You can, and should ,modify this recipe by what your tastes call for.
Extra spicy? Add more peppers.
Extremely spicy? Add even more peppers (Habeneros perhaps?)
More sweet? Roast the tomatoes longer and remove the lime juice?
Take this base and make it as is (it's my favorite rendition at least) and do whatever you want to it to make it your own custom Salsa Roja Asada.
Step 1: Ingredients
7 Roma Tomatoes
3 Serrano Chili Peppers
1 White Onion
1-2 Limes (depending on taste)
4-5 Cloves of garlic
HEAVY handful of Cilantro
2tsp of salt
5-6 grinds of fresh black pepper
drizzle of olive oil