Heat 1 tablespoon olive oil over medium-high heat in a 12-inch oven-proof skillet. Add 1/2 yellow onion and sauté for approximately 3 minutes or until soft. Add 1/2 red bell pepper, 15 ounces canned black beans, and 2 cloves garlic, and sauté for 2 more minutes.
Stir in 10 ounces Rotel tomatoes, 10 ounces frozen chopped spinach, 2 teaspoons chili powder, 1 teaspoon ground cumin, 1 teaspoon salt, 1/2 teaspoon smoked paprika, 1/4 teaspoon dried oregano, 1/4 teaspoon cayenne pepper, and 1/3 cup mayonnaise until well combined, and the spinach is warmed through.
Reduce the heat to low and stir in 4 ounces cream cheese until melted, followed by ½ cup sharp cheddar cheese until melted, and ½ cup pepper jack cheese until melted. Stir in1/3 cup sour cream until well combined.
Top the dip with the remaining ½ cup sharp cheddar cheese and ½ cup pepper jack cheese. Transfer to the oven and bake for 20 minutes. Broil for 1-3 minutes, watching closely, just until the cheese is golden and bubbly.
Carefully remove from the oven and top with avocado, tomatoes, and cilantro or parsley for garnish if desired. Serve warm with tortilla chips.
Notes
For the thickest, creamiest dip, thaw the spinach completely and squeeze out as much liquid as possible before adding it to the skillet. Softened cream cheese will melt more smoothly into the dip.
If you don't have an oven-safe skillet, prepare the dip on the stovetop and transfer it to an oven-safe baking dish before topping with cheese and baking.
You can substitute low-fat mayonnaise, 1/3 less-fat cream cheese, and reduced-fat sour cream if desired.