Heat oven to 425°F; brush 18-by-13-inch rimmed sheet with 1 tablespoon oil. Mix chicken, corn, dry spinach, cheese, jalapeños, cumin, garlic powder, and salt.
Overlap 6 tortillas around edges, half hanging over rim, to cover sides. Put 1 tortilla in center to cover bottom gap.
Spread filling evenly. Put last tortilla over center; fold overhanging halves inward to enclose.
Brush top with remaining oil; foil-cover. Set second clean sheet directly on top as light weight; bake 20 minutes.
Remove top pan and foil; bake 8 to 10 minutes more until golden and center reaches 165°F.
Rest 5 minutes; cut 12 rectangles. Serve 2 per portion with 2 tablespoons salsa.