Combine flour, cornstarch, onion powder, garlic powder, salt, black pepper, and cayenne pepper in a large shallow dish; set aside 1/3 cup in a small bowl. Gently press steaks in remaining flour mixture until well coated.

Melt 1 tablespoon butter with 1 tablespoon oil in a large skillet over medium heat. Add steaks; pan-fry until golden, about 4 minutes per side, flipping once. Transfer steaks to a platter.

Melt remaining 1 tablespoon butter with remaining 1 tablespoon oil in the skillet over medium heat. Add onion; cook and stir until translucent, 5 to 7 minutes.

Sprinkle reserved 1/3 cup flour mixture over onions; cook and stir about 1 minute. Stir in stock and milk; bring to a boil, stirring constantly, until gravy thickens, scraping the browned bits of food off the bottom of the skillet with a wooden spoon.

Return steaks to the skillet, submerging them in gravy. Cover the skillet, reduce heat to low, and simmer, stirring occasionally, until steaks are very tender, about 45 minutes.