Cook sausage and onion in a large skillet over medium-high heat 5 minutes, stirring until sausage crumbles and is no longer pink. Drain.
Combine cornmeal, flour, salt, and baking soda.
Stir together eggs, buttermilk, and oil until combined. Add to dry ingredients, stirring just until moistened. (Batter will not be smooth.) Add sausage mixture, peas, and remaining ingredients to batter, stirring well.
Pour into a greased 13- x 9-inch baking dish.
Bake at 350° for 1 hour or until golden and set.
Note: Freeze baked cornbread up to 1 month, if desired. Thaw in refrigerator overnight. Bake, covered, at 350° for 30 minutes. Uncover and bake 10 more minutes or until thoroughly heated. To reheat directly from the freezer, bake, covered, at 350° for 1 hour. Uncover and bake 10 more minutes or until thoroughly heated.
To lighten: Substitute 1 (12-ounce) package reduced-fat sausage, fat-free buttermilk, 1/2 cup egg substitute, and 2% reduced-fat Cheddar cheese. Reduce oil to 2 tablespoons. Prepare and bake as directed.