In saucepan, simmer lime juice, water and sugar until slightly thickened, 6 minutes (will be about 1/2 to 2/3 cup). Remove from heat.
For sauce, bring large pot of water to boiling. Add chard; cook just until wilted, 1 minute. Drain.
Season fish with 1/2 teaspoon salt and the pepper. Dust with Wondra.
Heat large skillet over medium-high heat. Add oil, tilting pan to coat. Add 4 fillets; cook until slightly golden and fish begins to turn white, 1 to 2 minutes. Flip fillets over; cook 1 minute. Transfer to platter; cover with foil. Keep warm in 200° oven. Repeat with remaining fillets.
Add butter to skillet. Reduce heat to medium. Add leek; cook until softened, 5 minutes. Add wine; cook 2 minutes. Add 1/3 cup sauce; cook 1 minute. Add beans and greens; toss to coat. Season with remaining 1/4 teaspoon salt. Remove from heat.
Divide bean mixture among 4 plates; top with fillets and sauce.