Heat the butter and oil in a large skillet over medium-low heat. Add the onion and garlic and cook until fragrant and the onion is translucent, about 6 minutes.
Add the jalapeños, squash, tomatoes and 2 tablespoons water. Stir in the oregano and 1 teaspoon each salt and pepper. Cover and cook, adding a couple more tablespoons of water about halfway through if the squash seems to be sticking to the bottom of the pan, until the squash is tender, 17 to 20 minutes.
Serve in a bowl with a dollop of sour cream and a sprinkle of crumbled queso fresco. Top with more oregano if desired.
