Assemble the sandwiches. On four slices of bread, layer half the spinach, the tomato slices, salt, pepper, red onion, and fresh basil.
Sprinkle evenly with feta cheese.
Top with the remaining spinach and the other four slices of bread, pressing down firmly.
Prepare your cooking surface. Lightly coat a sandwich press, indoor grill, or large nonstick skillet with cooking spray and preheat over medium heat.
Grill the sandwiches in batches if needed. For a sandwich press, cook for 2-3 minutes until the bread is golden and the cheese has melted. For a skillet or grill pan, cook for 1-2 minutes per side, pressing down with a heavy pan or spatula, until toasted and the cheese is melted.
Add a flavor boost by spreading a thin layer of pesto or hummus on the bread before adding the other .
Don't be shy with the spinach! It wilts down significantly when cooked, so pack it in for extra nutrients and flavor.