Inverted Nachos Casserole of Beer Braised Black Beans and Mushrooms

Name: Inverted Nachos Casserole of Beer Braised Black Beans and Mushrooms
Ingredients: 2 T vegetable oil; 2 medium onions (chopped); 6 cloves garlic (minced); 16 oz sliced crimini mushrooms; 1/2 t Mexican oregano; 2 t ground cumin; 1 t ancho chile powder; 2 sweet bell peppers (chopped); 1 12-oz bottle lager beer (divided); 2 cups cooked black beans; 2 cups cooked kidney beans; 2-3 oz monterey jack cheese (shredded); 2-3 oz cheddar cheese (shredded, divided); several large handfuls of crumbled tortilla chips; sour cream and salsa for serving