Mexican cuisine is, objectively, the best of the international foodstuffs. It's so vitally good that UNESCO added it to the Representative List of the Intangible Cultural Heritage of Humanity in 2010.
That's why it's a fucking shame that Glorious Leader Trump is going to round up and deport all of the Mexicans next year, and then make them build their own wall. And since tamales and moles and the like can be (kind of) a pain in the ass to make, we'll have to learn to make salsa if we ever want to experience joy again. You will need:
A word on tomatoes: don't buy them at the grocery store because grocery store tomatoes fucking suck. Even fresh, even when they're in season (which is now). If you read this website, odds are you have an annoying asshole foodie friend who grows their own on their balcony. If not, try your best to get up at the asscrack of dawn and make it to the farmer's market before the old people buy all the good shit.
As for the peppers, order them online in the ethnic section of your grocery store or in the bulk section at Whole Foods. To thine own self be true when it comes to spice. 4 peppers will make a spicy salsa, 2 will be mild-medium. You can't fucking use "none" and call it salsa.
To begin, get a large pan as hot as you've ever gotten anything (e.g. hotter than a West Virginian girl's curling iron on prom night). DO NOT use a nonstick pan for this, as the heat we're dealing with will release aerosolized cancer and kill everyone in your building. A well-seasoned cast iron is best, by a mile. If you don't have that, stainless steel will do. Whatever you're working with, get it over your very highest heat, and work on the ingredients.
Remove the tomatoes from the vine (if there is one) and use a paring knife to kind of extract the shit on top (this is annoying and you'll prob cut yourself). Then, bisect them along their equators. For the onion, first cut it in half from pole to pole, then take one of those halves and slice it into disks as thick as your middle finger. That's it, no more cutting!
Your pan should be smoking FURIOUSLY by now, which means it's time to roast this shit. What I like to do is start by arranging the tomatoes, cut sides down, in a circle around the perimeter. If there's room in the middle, start laying down your onion slices. Then, don't do shit for a while.
See, if you've ever bought one of those "salsa kits" from Trader Joe's and chucked it all into a blender only for it to taste like shit, it's because everything was raw. Good salsa benefits not just from having the ingredients cooked, but the hellish char that results from dry-roasting everything. So, leave the shit alone for like, a while. 8 minutes? 10 minutes? After some time has passed, use tongs to gently check the tomatoes, looking for a) an easy release from the pan and b) significant blackening. If you're there, flip everything and wait some more.
Once all your shit is blacker than your soul, do the rest of the stuff. Pop the garlic cloves, on there, whole, giving them a minute or so to darken up. With the peppers, lay them on the smoldering pan just until they puff up a little. That's it. Get everything into the blender, and then go get very, very drunk.
No, seriously. This shit is hotter than your little sister, and blending it now will cause it to explode and scald you and make you ugly so that no one will want to share your salsa with you. Give it, oh, two hours. Then come back, toss in a big pinch of salt, a splash of cheap white vinegar, and pulse it up in the blender. You're not making pico here so do blend it some, but it'd be a shame to blend it into full on "restaurant style" watery bullshit. Just eyeball it.
That's it! Put your salsa on, uh, everything you put salsa on now, you dingus. This makes about 1.5 quarts of salsa, so I recommend keeping half in the fridge, and saving half to rub in Donald Trump's underpants at your nearest convenience.