Calabacitas a la Mexicana

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1 tablespoon unsalted butter

1 tablespoon canola oil

1/2 medium white onion, diced (about 1/2 cup)

1 teaspoon minced garlic

2 jalapeños, seeded, deveined and finely chopped (about 1/4 cup)

2 summer squash or zucchini (about 12 ounces), diced (about 3 cups)

2 plum tomatoes, diced (about 1 1/4 cups)

2 teaspoons dried Mexican oregano, plus more for serving, optional

Kosher salt and freshly ground black pepper

Sour cream, for serving

Crumbled queso fresco, for serving