In a large skillet, heat the olive oil over medium-high heat. Add the onion and a pinch of salt and cook until softened and lightly browned, about 4-5 minutes, stirring frequently. Stir in the corn and cook until lightly browned, about 4 minutes. Stir in the rice, garlic, cumin and cayenne pepper. Cook for 30 seconds, or until just fragrant, stirring constantly. Add the broth to the pot. Bring to a low simmer and cook over low heat, covered, for 25 minutes. Stir every 5-10 minutes.
Add the black beans to the pan. Cover and simmer until the liquid is fully absorbed, about 20-25 minutes. Season to taste with salt and black pepper.
Meanwhile, toss the heirloom tomatoes, scallions, cilantro and lime juice together in a large bowl. Season to taste with salt and black pepper.
Serve the rice and beans topped with the heirloom tomato salsa.