Use a fine sieve mesh strainer to rinse the rice, and then allow it to drip dry.
In a large skillet with at least 2'' sides and fitted with a lid, heat the oil over medium high heat.
Add the rice and cook, stirring occasionally, until the rice is golden brown. (This process takes about 10 minutes to get the rice really golden...be sure to keep an eye on it and stir it often enough that it doesn't burn).
Once the rice is toasted, add the onion, zucchini, squash, bell pepper, black beans, corn, one diced roma tomato, and garlic.
Stir well and saute for 1-2 minutes. Chop the tomato bullion cubes into pieces and add to the pan.
Add salsa, chicken broth and water and season with a little salt. Bring mixture to boil.
Cover, reduce heat to low, and cook for 20 minutes.
Sprinkle cheese, cilantro, remaining diced tomato, and avocado over the top.
You can serve it as is, or with tortillas, over lettuce/salad, or over tortilla chips.
Notes
*Tomato bullion cubes can be found in the Mexican aisle at your grocery store. They're really inexpensive and sometimes the box is in Spanish-- called "Caldo de Tomate"