To prepare succotash, heat a large nonstick skillet coated with cooking spray over medium-high heat. Add the onion; sauté 5 minutes or until tender. Add garlic and jalapeño; sauté 30 seconds. Add zucchini; sauté 5 minutes or until zucchini is crisp-tender. Stir in corn; sauté 5 minutes or until corn is crisp-tender. Remove from heat; stir in chile powder, 1 teaspoon salt, and black pepper. Combine the zucchini mixture, cheese, and beans in a 13 x 9-inch baking dish coated with cooking spray.
Preheat oven to 375°.
To prepare topping, lightly spoon the flours into dry measuring cups; level with a knife. Combine flours, baking powder, oregano, baking soda, and 1/4 teaspoon salt in a medium bowl, stirring with a whisk. Cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Combine milk and yogurt; add milk mixture to flour mixture, stirring just until moist. Drop dough by tablespoonfuls onto zucchini mixture. Brush beaten egg over topping. Bake at 375° for 30 minutes or until topping is golden.