In a large skillet over medium-high heat, add the olive oil and, when hot add the onions and cook, stirring occasionally, until golden, 8 to 10 minutes. Add the chorizo and cook, stirring occasionally, for 5 minutes. Add the garlic and saute for 1 minute. Add the potatoes, oregano, salt, and cumin and cook, stirring occasionally, for 10 minutes, or until potatoes are golden brown. Set aside.
Preheat a grill to medium-high and line a large baking sheet with parchment paper. Preheat the oven to 400 degrees F.
Spread 1/2 cup of the cheese evenly over each of 2 tortillas. Top each tortilla with 1 of the remaining tortillas to form 2 sandwiches and place these on the grill. Grill, turning often and pressing with a spatula to remove any air pockets, for 12 to 15 minutes, or until golden brown and crisp on both sides. Transfer to the prepared baking sheet and top each of the "pizzas" with 2 tablespoons of the salsa, spreading with the back of a spoon. Divide the chorizo-potato topping evenly between the pizzas and sprinkle the remaining cheese evenly over the tops. Bake for 12 to 14 minutes, or until the cheese is melted and golden brown. Slice into wedges and serve, garnished with sour cream, green onions, cilantro and roasted red pepper slices.