Heat 2 tablespoons olive oil in a cast-iron or other heavy grill pan over medium high heat. Add the onions. As soon as they start to sizzle, reduce heat to very low, cover the skillet, and cook the onions until soft and lightly browned, about 12 minutes. Add the garlic and stir, then add the spinach and cook until wilted. Add the balsamic vinegar and orange juice to the skillet, stir and cook another few minutes. Season lightly with salt and pepper. Remove and cool slightly. Wipe the skillet clean.

Using a serrated knife, split the bread in half or have two slices of sourdough bread ready per panini. Layer the mozzarella, spinach-onion-garlic mixture, basil and tomatoes over the bottom half of the bread. Cover with top slice.

Heat another 2 tablespoons of olive oil in the grill pan. Press down and place in the grill pan. Grill until the bread is golden and the cheese melts, about 4 to 5 minutes per side, pressing the sandwiches with a large spatula to flatten them as you grill them. Remove and serve.