How to Make It
Mix masa harina and salt in a large bowl. Slowly add the water while mixing the ingredients with your hand. Knead mixture until all water is incorporated; dough will be sticky. Let rest about 5 to 10 minutes.
Combine turkey, mozzarella, cumin, and garlic in a separate small bowl.
With damp hands, roll dough into 2-inch balls (slightly larger than a golf ball). Hold one ball in a cupped hand and, with your other hand, press your thumb into the ball to make a large well for the filling. Add a heaping tablespoon of turkey mixture to the well and bring dough around filling to enclose. Gently shape dough back into a ball. Flatten into a disc about 4-inches in diameter. Repeat with remaining dough and filling; dampen hands as needed.
Heat oil in a large skillet over medium. Gently place pupusas in skillet and cook until golden brown in spots, 3 to 4 minutes per side. Repeat with remaining pupusas, adding more oil if needed. Drain on paper towels.
Serve with salsa verde, sour cream, and pickled red onions.