Chipotle Cauliflower Tostadas

Name: Chipotle Cauliflower Tostadas
Ingredients: 7 tablespoons extra-virgin olive oil, plus more for frying; 1 large white onion, 1/2 chopped and 1/2 thickly sliced; 1/2 head cauliflower, finely chopped (about 2 cups); Kosher salt; 1 canned chipotle in adobo sauce, plus 2 teaspoons sauce; 1/2 teaspoon dried oregano; 1 15-ounce can pinto beans, drained and rinsed; 8 corn tortillas; 1 8-ounce package portobello mushroom caps; 2 large poblano peppers, stemmed and seeded; Chipotle salsa and chopped fresh cilantro, for serving