Cook the rice according to package directions. Note:If leftover plain brown rice is available to use, skip Step 1. Meanwhile, simmer beans in their liquid with 1/2 teaspoon salt for 5 minutes, then drain beans.
In a large skillet over medium heat, add the oil and cook the onion until softened and beginning to lightly brown at the edges. Add garlic, coriander, cumin, and oregano; stir until fragrant. Add the stock, tomato paste, tomatoes, and 1/4 teaspoon salt, stirring to loosen any bits and spices from the pan. Simmer 2 minutes to allow liquid to evaporate. When the rice is done, stir the tomato mixture into the rice, cover, and set aside.
In the same skillet, heat enough oil to coat the bottom of the pan over medium-high heat. Add the cauliflower to the skillet and season with ancho chile powder, salt and pepper. Cook, stirring occasionally, until almost soft, about 3 minutes. It's okay if it browns in spots, but important that it doesn't overcook in order to retain a firmer texture. Add corn and beans to the skillet with the cauliflower to warm through, about 1 minute. Transfer to a large mixing bowl and toss with lime zest, rice mixture, and cilantro.
Toast the pepitas over med-high heat in a dry skillet until browned and popping, about 2 minutes (this can be done in advance). Stir into rice just before serving.
Roasted and salted pepitas (pumpkin seeds) can be substituted for the raw pepitas, just reduce the amount of salt in the recipe by 1/4 teaspoon and skip the step of toasting in the skillet.