Place the 8 ounces cream cheese and ¼ cup sour cream in a large bowl. Beat with a hand mixer until smooth.

Stir in ½ cup of the grated Havarti, ¼ cup grated Parmesan, 2 cloves minced garlic, a pinch of red pepper flakes, ½ teaspoon kosher salt, and ¼ teaspoon freshly ground black pepper until well combined.

Add the defrosted, well–squeezed dry 9-ounce package of chopped spinach and the drained, chopped 14-ounce can of artichoke hearts. Fold until evenly mixed. Set the spinach-artichoke mixture aside.

Reserve the remaining grated Havarti (the portion not added to the cream cheese mixture) for assembling the quesadillas.

Heat a large skillet over medium heat and melt the 1 tablespoon butter or heat the 1 tablespoon olive oil in the pan.

To assemble each quesadilla, lay one street taco tortilla in the skillet, sprinkle some of the reserved Havarti evenly over the tortilla, then spoon about ¼ cup of the spinach-artichoke mixture onto the cheese, spreading it slightly but keeping it away from the edges.

Top with a second tortilla and press down gently with a spatula to flatten the filling and help the tortillas make contact with the pan.

Cook until the bottom tortilla is golden brown and the cheese begins to melt (about 1–2 minutes), then carefully flip the quesadilla and cook the other side until golden and the filling is heated through (about 1–2 minutes more).

Transfer the cooked quesadilla to a cutting board and cut in half if desired. Repeat steps 6–8 with the remaining tortillas, reserved Havarti, and filling.

Serve the quesadillas warm, with salsa if desired.