Preheat oven to 425 degrees.
Toss 1 teaspoon kosher salt with 1 tablespoon of chipotle chili powder and the lime zest. Set aside.
Rub both sides of the tortillas lightly and place on a large baking sheet. I fit 4 at a time on my baking sheet, so you will likely need to do two batches. Bake for 8-9 minutes until crisp. Sprinkle each tortilla with the chili lime salt while they're still warm, then transfer to a wire rack. Repeat with another batch of tortillas.
In a large bowl, stir together the black beans, cilantro, lime juice, shallot, garlic, and the remaining 2 teaspoons of chipotle chili powder. Season with salt and pepper to taste.
To make the avocado crema: add the avocado, yogurt, lime juice, and a pinch of salt to a large food processor. Blitz until very creamy and smooth, scraping down the sides as needed with a spatula, about 1-2 minutes. Taste and adjust salt as needed.
Spread the tostadas with the avocado crema, then spoon black beans on top. Arrange pickled onion, cilantro, the serrano/jalapeno slices, and yogurt/sour cream over the black beans. Finish with a squeeze of lime juice. Serve immediately.