In a large mixing bowl, combine the all-purpose flour, whole wheat flour, sugar, garlic salt, and yeast. Stir until well combined. Gradually add the very warm milk and 1/4 cup of olive oil to the dry ingredients. Mix until a dough forms.
Turn the dough onto a lightly floured surface and knead for about 5-7 minutes until it is smooth and elastic. Place the dough back into the mixing bowl, cover it with a kitchen towel, and let it rise in a warm area for about 30 minutes or until it has doubled in size.
While the dough is rising, heat 1 tablespoon of olive oil in a frying pan over medium heat. Add the sliced mushrooms and sauté for about 5 minutes until they are tender. Add the fresh spinach and cook until wilted, about 2 minutes. Remove from heat and set aside.
Once the dough has risen, preheat your oven to 425°F (220°C). Grease your 9x13 inch baking dish with cooking spray or a bit of olive oil. Press the dough into the bottom of the prepared dish, spreading it evenly.
Layer the sautéed mushrooms and spinach mixture over the dough. Sprinkle the crumbled feta cheese, chopped roasted red bell peppers, and Kalamata olives evenly on top. Finally, add the shredded mozzarella cheese and sprinkle the Italian seasoning over everything.
Place the One-Dish Greek Pizza in the preheated oven and bake for 20-25 minutes or until the cheese is bubbly and golden brown. Remove from the oven and let it cool for a few minutes.
If desired, garnish with fresh basil leaves before slicing. Serve warm and enjoy the delicious flavors of Greece in every bite!