Grease an 8 x 8-inch baking dish.
In a mixing bowl, combine 1 cup all-purpose flour, 1/2 cup whole wheat flour, 1 tablespoon sugar, 1/2 teaspoon garlic salt, and 2 envelopes RapidRise yeast; stir to blend.
Add 3/4 cup very warm milk (120–130ºF) and 1/4 cup olive oil to the dry ingredients; mix until a batter forms, then gently spread or press the batter into the bottom of the prepared baking dish.
Evenly sprinkle the batter with the 4 ounces crumbled feta and 2 cups packed fresh spinach leaves.
Heat 1 tablespoon olive oil in a skillet over medium-high heat and sauté the sliced mushrooms for about 2 minutes until just softened.
Remove the skillet from heat and stir in 1/2 cup chopped roasted red bell peppers and 1/4 cup chopped Kalamata olives.
Spread the mushroom, pepper, and olive mixture evenly over the feta and spinach layer.
Sprinkle 1-1/2 cups (about 6 ounces) shredded mozzarella evenly over the top and dust with 1/2 teaspoon Italian seasoning.
Place the dish in a cold oven, set the oven to 350°F, and bake for 30–35 minutes or until the center is set and the top is golden.
Remove from the oven and, if desired, top with fresh basil before serving.