Whisk together the cumin, garlic, lime juice, orange juice, 1/2 teaspoon salt and a few grinds of pepper in a large bowl and add the steak. Mix well to coat and let marinate for 10 minutes.
Lightly oil the grill grates and cook the steak, flipping once, until medium rare, 2 to 6 minutes per side depending on the thickness. Remove the steak to a cutting board and let rest 5 minutes. Slice the steak against the grain and then into bite-size pieces.
Lightly brush the zucchini with oil and grill, flipping as needed, until tender and lightly charred, 4 to 6 minutes. Season with salt and pepper. Cut into bite-size chunks and set aside to cool slightly.
Lay a tortilla on a work surface and sprinkle evenly with 1/2 cup of the cheese. Arrange one-quarter of both the steak and zucchini on top of the cheese. Fold the tortilla in half to enclose the fillings. Repeat with the remaining tortillas, cheese, steak and zucchini.
Melt 1/2 tablespoon butter in a large nonstick or cast-iron skillet over medium heat. Add a quesadilla and cook, flipping once, until golden brown on both sides and the cheese is melted, about 3 minutes per side. Remove to a cutting board and repeat with the remaining butter and quesadillas.
Cut the quesadillas into triangles and serve with the chopped scallions, sour cream, guacamole and salsa.
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