Clean and peel poblano chiles (if you are not familiar with the process, check out how to do it right here), remove seeds, then stem and cut in long strips. Set apart.
Warm up butter, add onion, salt and pepper to taste and cook until onion is soft, approx 5-7 minutes. Add poblano chiles and cook for one more minute.
Turn off the heat, add cream and adjust salt.
Take a tostada, spread warm refried beans over it, add the poblano chile and onion mixture, then top with queso fresco or queso panela and a salsa of your choice.