Put water in vegetable steamer and place it over high heat. Add bok choy to steamer and let steam until tender, about 4-5 minutes.
Coat a large nonstick skillet with cooking spray and set over medium hear. Add cumin and red pepper flakes; cook 2 minutes.
Salt fish and add to skillet; cook 2 - 3 minutes per side, until firm and opaque.
To serve, divide bok choy among 4 plates, sprinkle with salt and lemon juice; top with mahi mahi. Yields about 1 cup bok choy and 1 fillet per serving.