Prep the veggies: Wash and dry all produce. Preheat the oven to 425 degrees. Drain and rinse the beans. Halve, peel, and thinly slice the onion. Finely chop the cilantro. Halve the tomatoes.

Cook the beef: Heat a drizzle of olive oil in a large pan over medium-high heat. Add the onion to the pan and cook, tossing for 3-4 minutes, until softened. Add the ground beef and oregano to the pan and cook, breaking up the meat into pieces, until browned and cooked through. Season with salt and pepper.

Assemble and bake the quesadillas: Rub one side of each tortilla with a small drizzle of olive oil and place oil-side down on a baking sheet. Sprinkle half of each tortilla with some mozzarella cheese. Top with the ground beef and another sprinkling of mozzarella cheese. Fold over the other side of the tortilla and repeat for the remaining quesadillas. Bake for 7-10 minutes, flipping halfway through cooking, until melted and crispy.

Make the salsa: Meanwhile, halve, pit, and dice the avocado. In a medium bowl, combine the avocado, black beans, tomatoes, cilantro, and a squeeze of lime juice. Season to taste with salt and pepper.

Finish and serve: Cut the quesadillas into wedges and serve with a dollop of sour cream and a side of avocado-black bean salsa.