For the onion rings: Preheat the oven to 425 degrees F. Place 3 shallow bowls on the counter in a row with a foil-lined baking sheet fit with a cooling rack at the end.
Add 1 cup of the flour to one bowl, the egg whites mixed with 1/4 cup water and 1 teaspoon flour in the next bowl, and the breadcrumbs whisked with the cayenne in the last. Sprinkle each bowl with salt and black pepper.
Begin by tossing an onion ring in the flour. Dip into the egg mixture, followed by the breadcrumbs, and then place on the rack. Repeat with the remaining onion rings.
Spray all of the breaded onion rings with canola spray and sprinkle with salt and black pepper. Bake until the onion rings are crispy, 15 minutes.
For the dipping sauce: Meanwhile, whisk together the yogurt, olive oil, lime zest and juice, chipotle and some salt and black pepper.
Serve the onions rings with the dipping sauce.