Thinning Tomato paste

There's a few types of salsa -- salsa fresca (aka pico de gallo, aka salsa cruda), which is "fresh salsa" and uncooked, and if made fresh, it shouldn't be too watery (unless you add to much liquid, eg, lime juice), but letting the vegetables sit after salting will start to draw out extra liquid and could become watery.

For truly thick salsa, you have to treat it like a jam, and cook it to release the pectin in the tomato. (or I guess you could use some other thickeners ... I've never tried. tomato paste, maybe?) You don't necessarily have to cook the other ingredients, too, but I personally like roasting the peppers and onions to sweeten then up and remove some of the extra liquid. (halve the peppers and seed them, lay them on a tray cut side up with the onion cut into wedges, roast 'til they're softened, then dump into a food processor and either pulse or liquify it, depending on how chunky you like it. And the skin falls off the peppers, so I leave 'em out).

You can do the same roasting with the tomatoes, and it'll be thicker from the food processor, but you won't get the same cling without letting them cook slowly in liquid to develop the pectin. If you roast the tomatoes, I like to use plum tomatoes, as they have more "meat" to the gel around the seeds, and take out the gel and seeds.

Tomato paste is just tomatoes with the water removed, essentially. I'd slice the tomatoes in half and roast them (cut side up) at 350 degrees F for an hour (this will concentrate the flavour nicely and you can add s&p/olive oil/herbs/garlic if you want). Then mash them through a sieve or food mill to get a smooth consistency. Then put that tomato puree in a pot and just boil them down until it's as thick as you want. As it gets thicker, you'll need to stir it regularly to prevent it sticking to the bottom of the pot and burning.