Preheat your oven to 425°F (220°C) and cut the cauliflower into small, even florets. Toss with olive oil, salt, and pepper, then spread on a baking sheet lined with parchment paper.

Roast the cauliflower for about 20 minutes, flipping halfway through until they are golden and tender.

Heat oil in a frying pan over medium-high heat and fry the corn tortillas for 1-2 minutes on each side until golden and crispy. Drain on paper towels.

In a small bowl, combine sliced red onions, vinegar, and salt. Let them sit for 10-15 minutes to pickle.

Assemble the tostadas starting with a tortilla base, spreading black beans, followed by roasted cauliflower, and drizzling with queso.

Top each tostada with pickled onions for added flavor and crunch. Serve immediately while warm and crispy.