Preheat oven to 200°F. Place a wire cooling rack on a rimmed baking sheet and set in oven.
Mix flour, 5 teaspoons salt, 1 teaspoon pepper, and cayenne pepper in a shallow dish for dredging.
In another dish, whisk together beaten eggs, baking powder, baking soda, and buttermilk until well combined. Some foaming is normal.
Pat each steak dry with paper towels. Dredge steak in flour mixture, coating both sides evenly.
Using tongs, transfer steak to egg mixture, coating both sides thoroughly.
Return steak to flour mixture for a second coating on both sides. Place on a wire rack.
Repeat process with remaining steaks.
Heat peanut oil in a deep pan or Dutch oven to 375°F, maintaining 1-inch depth.
Fry steaks for 2½ minutes per side until deep golden brown. Maintain oil temperature at 375°F between batches.
Briefly drain steaks on paper towels, then transfer to oven rack to keep warm.
Strain frying oil, reserving 2 tablespoons. Return oil crumbs to the pan with reserved oil.
Sauté minced onion until brown, add garlic and cook 30 seconds more.
Whisk in 3 tablespoons flour and cook until lightly browned.
Gradually whisk in chicken broth, then milk, salt, and pepper.
Bring to a gentle boil, then simmer until thickened.
Serve steaks immediately with gravy spooned over top.