Preheat oven to 350°F. Grease an 8 x 8-inch baking dish.
In a medium bowl, whisk together 1 cup all-purpose flour, ½ cup whole wheat flour, 1 tablespoon sugar, ½ teaspoon garlic salt, and both envelopes (2) Fleischmann's® RapidRise™ Yeast until evenly combined.
Add ¾ cup very warm milk (120º to 130ºF) and ¼ cup olive oil to the dry ingredients. Stir until a smooth, spreadable batter forms.
Pour the batter into the prepared baking dish and gently press/spread it into an even layer covering the bottom.
Evenly sprinkle the 4 ounces crumbled feta cheese over the batter. Top the feta with the 2 cups packed fresh spinach leaves.
In a skillet, heat 1 tablespoon olive oil over medium-high heat. Add the 8 ounces sliced fresh mushrooms and sauté for about 2 minutes, stirring, until the mushrooms begin to soften.
Remove the skillet from the heat and stir in ½ cup chopped roasted red bell peppers and ¼ cup chopped Kalamata olives. Mix to combine.
Spread the mushroom, pepper, and olive mixture evenly over the spinach layer in the baking dish.
Sprinkle 1-1/2 cups (6 ounces) shredded mozzarella cheese over the top, then evenly dust with ½ teaspoon Italian seasoning.
Bake in the preheated 350°F oven for 30–35 minutes, or until the pizza is baked through, the center is set, and the top is lightly golden.
If desired, top with fresh basil just before serving. Allow to rest briefly, then slice and serve.