In a medium saucepan, combine the rinsed rice, vegetable broth, and salt. Bring to a boil, then reduce heat to low, cover, and simmer for 15-18 minutes, or until all liquid is absorbed and rice is tender.

Remove from heat and let stand, covered, for 5 minutes. Fluff with a fork, then stir in the fresh lime juice and chopped cilantro. Set aside.

Heat olive oil in a large skillet over medium heat. Add the chopped yellow onion and cook for 3-4 minutes until softened and translucent.

Add the minced garlic and diced bell pepper to the skillet. Cook for another 3-5 minutes until the bell pepper is tender-crisp.

Stir in the rinsed and drained black beans, frozen corn, cumin, smoked paprika, chili powder, and cayenne pepper (if using). Pour in the vegetable broth or water.

Bring to a gentle simmer and cook for 5-7 minutes, stirring occasionally, until the liquid has mostly reduced and the flavors have melded. Season with salt and black pepper to taste. Remove from heat.

Warm the flour tortillas either in a dry skillet over medium heat for 15-30 seconds per side, in the microwave for 15-20 seconds, or wrapped in foil in an oven at 300°F (150°C) for 5-10 minutes. Warming makes them more pliable and easier to roll.

Lay each warm tortilla flat. Spoon about 1/4 of the lime-cilantro rice onto the bottom third of the tortilla, leaving space around the edges.

Top the rice with 1/4 of the black bean and corn filling. Add a few slices of avocado or a spoonful of mashed avocado. If desired, drizzle with vegan sour cream and a spoonful of salsa.

Fold the sides of the tortilla inwards over the filling.

Starting from the bottom edge closest to you, tightly roll the tortilla upwards, tucking in the filling as you go, until you have a compact burrito.

Repeat with the remaining tortillas and filling.

Serve immediately, optionally garnished with extra fresh cilantro, salsa, or a side of tortilla chips.