Preheat oven to 400 degrees F (200 degrees C). Line a baking sheet with parchment paper.
Place onion slices in a small colander. Bring 2 to 3 cups water to a boil in a kettle. Pour over onion and let drain.
Mix vinegar, salt, sugar, and cumin together in a jar. Mix in onion slices. Cover and place in the refrigerator.
Cut cauliflower in half from top to bottom. Place each half flat against the cutting board. Cut into 1/2-inch slices. Arrange carefully on the prepared baking sheet, trying to keep slices intact. Drizzle olive oil over cauliflower; season with salt and pepper.
Roast cauliflower in the preheated oven until evenly browned, about 30 minutes.
Combine avocado, sour cream, lime juice, 1 tablespoon cilantro, and salt in a blender. Blend until creamy and smooth.
Heat black beans in a small saucepan until heated through, about 5 minutes.
Spoon warmed beans over cauliflower. Dollop avocado crema over beans. Scatter pickled onion slices, cherry tomatoes, and queso fresco over crema. Garnish with 2 tablespoons chopped cilantro. Serve with lime wedges.