Position an oven rack in the middle of the oven and preheat the broiler.
In a 12-inch ovenproof nonstick skillet or a very well-seasoned cast-iron skillet, heat the olive oil and butter over medium-high heat. Add the potatoes, garlic, onions, red and orange bell peppers and jalapeno to the skillet. Add the cumin and season with salt and pepper. Cook, stirring gently, until the veggies begin to soften, about 5 minutes. Arrange the veggies so that the potato slices are flat and in an even layer.
Meanwhile, in a pitcher or bowl, whisk together the eggs, salsa and a pinch of salt. Stir in the grated pepper jack.
Pour the egg mixture into the skillet, being careful to cover the veggies and get it around the edge of the skillet. Cook, undisturbed, until the eggs begin to set on the bottom, about 5 minutes. Transfer the skillet to the broiler, using the middle rack, and cook until very lightly browned on top, about 5 minutes.
Carefully slide the frittata onto a cutting board. Slice into 4 to 6 wedges. Garnish with the cherry tomatoes, avocado, cilantro and a dollop of sour cream.