Separate sliced onions. Place in milk for 30 minutes with some ice cubes. (This will help ensure crispness.)
Heat oil to 365 F.
In a small bowl, whisk together the flour, baking powder and salt.
Remove onion rings from the milk and dip them into the flour mixture until all are well coated. Set onion rings aside.
Mix the beaten eggs and milk into the remaining flour mixture, blending with a fork. (You may need to adjust flour or milk quantity to create a pancake-like batter.) Dip the floured onion rings into the batter to coat and place on a wire rack. Place rack over a sheet of wax paper for easier cleanup.
Spread bread crumbs on a large plate or wax paper and place rings one at a time into the crumbs, scooping the crumbs over the rings to coat.
Repeat with remainder of the rings; fry a few at a time in hot oil, turning over until golden brown, 2-4 minutes. Drain on paper towels and serve, or keep warm in oven set at 250 F.