Preheat oven to 400 degrees Fahrenheit.Line a small baking sheet with parchment paper. Spread the tomato slices out across the baking sheet. Drizzle with olive oil and sprinkle with salt, pepper, and cumin.Roast tomatoes for 15 minutes. Remove from oven and let cool.While the tomatoes are roasting, smash the avocado in a small bowl. Add in the lime juice, salt, and pepper, to taste. Stir in the black beans.Slather the toasts with the avocado mixture and top each with 2 slices of roasted tomato. Sprinkle with pepitas and serve immediately.