Preheat the oven to 425F. Toss the plantains with the avocado oil and transfer them to a parchment or foil-lined baking sheet. Sprinkle them with salt. Bake for 25-30 minutes, flipping the rounds once halfway through cooking.
While the plantains bake, prepare the rice. Heat two teaspoons of the oil in a medium sized pot over medium low heat. Add the rice and toast, stirring constantly, for a few minutes, or until the rice is smelling nutty and fragrant. Add the salt and water and bring to a boil. Reduce to a simmer, cover, and simmer for 15 minutes, or until the rice has absorbed all of the liquid. (If you use brown rice, this will take more like 40-45 minutes, and you may want to start the rice before you bake the plantains.) Fluff the rice gently with a fork, cover, and allow it to steam for another 5-10 minutes, then stir in the remaining teaspoon oil, lime juice, and cilantro.
To assemble the bowls, place a quarter of the rice, a half cup of the black beans, and a quarter of the plantain rounds into each of four serving bowls. Top with chopped red onions, avocado slices, additional cilantro, and/or whatever other toppings you like. Serve.
Notes
Leftover beans will keep for up to 6 days in an airtight container in the fridge. Leftover rice will keep for up to 3 days. Plantains will keep for 2 days.