Grease an 8 x 8-inch baking dish.
In a medium bowl, combine 1 cup all-purpose flour, ½ cup whole wheat flour, 1 tablespoon sugar, ½ teaspoon garlic salt, and 2 envelopes Fleischmann's® RapidRise™ Yeast. Stir to blend.
Add ¾ cup very warm milk (120º to 130ºF) and ¼ cup olive oil to the dry ingredients and mix until a smooth batter forms.
Pour the batter into the greased baking dish and gently spread/press it evenly across the bottom.
Evenly sprinkle the 4 ounces crumbled feta cheese over the batter, then evenly scatter 2 cups packed fresh spinach leaves on top of the feta.
In a skillet, heat 1 tablespoon olive oil over medium-high heat. Add the 8 ounces sliced fresh mushrooms and sauté about 2 minutes until slightly softened. Remove the skillet from the heat and stir in ½ cup chopped roasted red bell peppers and ¼ cup chopped Kalamata olives.
Spoon and spread the mushroom–pepper–olive mixture evenly over the spinach layer.
Sprinkle 1½ cups (6 ounces) shredded mozzarella cheese evenly over the top and dust with ½ teaspoon Italian seasoning.
Place the assembled baking dish in a cold oven, set the oven to 350°F, and bake 30–35 minutes, or until the top is golden and the center is set.
If desired, top with fresh basil just before serving.