Start by rinsing and de-stemming the tomatoes. Roast them in a 400F oven for 20-25 minutes.
Roughly chop the 1/2 onion and saute in some oil over medium heat. Add a peeled, whole garlic clove to the pan. Saute until the onion starts to brown and then add the mixture to a blender.
Add the roasted tomatoes to the blender along with the onion mixture, 1/4 jalapeno, and 1/2 teaspoon of salt. Combine well. If you want more heat add another sliver of jalapeno.
Add a dollop of oil to a saucepan over medium heat. Add the tomato mixture from the blender and simmer for a few minutes. You can optionally add 1/2 cup of stock at this point -- this will keep the sauce liquidy. Take a final taste for seasoning, adding more salt if necessary.
Heat up the corn tortillas in the oven. You can alternatively give them a flash fry in some oil.
Use tongs to give the tortillas a swim in the sauce. Add the coated tortillas to a plate and fill with cheese. Other good options for fillings: refried beans, raw onion, shredded chicken, or your protein of choice.
Fold the tortillas (or roll them up) and coat them with plenty of the tomato sauce. You can optionally give them a few minutes in the oven to warm up.
Top them with a sprinkling of Cotija cheese, Mexican Crema, and finely chopped cilantro. Serve immediately.