To prepare dough, dissolve yeast in 3/4 cup warm water in a small bowl; let stand 5 minutes. Stir in 1 teaspoon oil. Lightly spoon flour into dry measuring cups; level with a knife. Combine flour and 1/2 teaspoon salt in a large bowl; add yeast mixture, stirring until dough forms. Turn dough out onto a lightly floured surface. Knead until smooth and elastic (about 6 minutes). Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, for 1 1/2 hours or until doubled in size. (Gently press two fingers into dough. If indentation remains, dough has risen enough.)
Punch dough down; cover and let rest 5 minutes. Roll dough into a 12-inch square; fit dough into an 8-inch square baking pan coated with cooking spray, allowing excess dough to hang over edges of dish.
Preheat oven to 425°.
To prepare filling, combine 1 teaspoon oil and 2 garlic cloves; set aside.
Heat remaining 2 teaspoons oil in a Dutch oven over medium-high heat. Add remaining 3 garlic cloves and onion; sauté 5 minutes or until onion is tender and lightly browned. Spoon onion mixture into a large bowl. Stir in 1/4 teaspoon salt and pepper; keep warm. Add half of spinach to pan; cook 1 minute or until spinach begins to wilt, stirring frequently. Add remaining spinach; cook 5 minutes or until spinach wilts. Place spinach in a colander; press until barely moist. Add spinach and cheese to onion mixture, stirring until well combined.
Brush dough with half of garlic-oil mixture; top with spinach mixture. Fold excess dough over filling to cover; brush with remaining garlic-oil mixture. Bake at 425° for 30 minutes or until golden. Let stand 10 minutes.
Wine note: Feta cheese is incredibly wine friendly, suitable with white, rosé, or even lighter reds. And a sauvignon blanc, like Chateau Ste. Michelle Columbia Valley ($10), is ideal with the Mediterranean flavors of Spinach-Feta Calzone Casserole. The wine's tangy acidity acts like a squeeze of lemon to punch up the flavors, while the salty cheese emphasizes the citrus fruit notes of the wine. --Jeffery Lindenmuth
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