1) Roast poblanos over an open flame on grill or gas stove until they are well-charred.
2) Add peppers to a large bowl, cover, and let steam for 10 minutes. Then peel peppers, cut off stems, cut out seeds, and slice thin.
3) Slice onions and add them to a medium pot with a drizzle of oil. Cover over medium heat until they start to soften, about 8 minutes.
4) Add minced garlic and spices to pot. Cook for another few minutes.
5) Add 1 cup of water to the pot and use the liquid to scrape off any stuck bits.
6) Simmer onions and spices until the water completely evaporates, about 8 more minutes.
7) Add drained beans and poblano peppers.
8) Stir in sour cream and cheese and season with salt.
9) To fry tortillas, add a drizzle of oil to a small pan and add a corn tortilla over medium-high heat. Cook for about 20 seconds on side one and then flip. Cook on the second side for about a minute and then flip back. The tortilla should be browned and starting to crisp. Remove it and let it drain on a paper towel. It will continue to crisp as it cools. Repeat with all the tortillas so you have crispy tostadas.
10) Serve poblano mixture over tostadas with toppings!