Adjust racks to top and middle positions and preheat oven to 450 degrees. Wash and dry produce.

Halve, core, and thinly slice bell pepper into strips. Peel and dice sweet potato into ½-inch pieces. Halve, peel, and thinly slice onion; mince a few slices until you have 2 TBSP (4 TBSP for 4 servings).

Toss bell pepper, sweet potato, and sliced onion on a baking sheet with a large drizzle of oil, Fajita Spice Blend, a big pinch of salt, and pepper.

Roast on top rack until browned and tender, 20-25 minutes. (For 4 servings, divide veggies between 2 sheets; roast on top and middle racks.)

While veggies roast, dice tomato into ¼-inch pieces. Roughly chop cilantro. Quarter lime. Mince jalapeño, removing ribs and seeds for less heat.

In a medium bowl, combine tomato, minced onion, cilantro, juice from half the lime, a pinch of salt and pepper, and as much jalapeño as you like.

In a small bowl, whisk together mayonnaise and guacamole. Whisk in water 1 tsp at a time until mixture reaches a drizzling consistency. Season with salt.

Drizzle tortillas with 1 TBSP oil (2 TBSP for 4 servings); brush or rub to coat both sides. Arrange on a second baking sheet in a single layer. Gently prick each tortilla in a few places with a fork.

Bake on middle rack, flipping halfway through, until lightly golden, 2-3 minutes per side. (For 4, wait for veggies to finish roasting, then transfer veggies to a large bowl. Divide tortillas between baking sheets and bake on top and middle racks, flipping tortillas and swapping rack positions halfway through.) TIP: Watch carefully—tortillas can quickly go from toasted to burned!

Meanwhile, heat a drizzle of oil in a medium pot over medium-high heat. Add refried black beans and cook until warmed through, 1-2 minutes.

Divide tostadas between plates; spread each with refried beans. Top with roasted veggies, salsa, and creamy guacamole. Serve with remaining lime wedges on the side.