ARRANGE racks in top and bottom thirds of oven. Preheat to 450F. Line 2 large baking sheets with foil.
BOIL vegetable broth and water in a medium saucepan. Stir in quinoa. Boil, covered, until quinoa is tender, stirring occasionally, about 12 min. Remove from heat and fluff with a fork.
CUT trout fillets in half crosswise. Arrange on prepared sheets. Sprinkle with 1/2 tsp salt. Season with fresh pepper. Roast in top and bottom thirds of oven, switching sheets halfway through, until a knife tip inserted into the thickest part of fish and held for 10 sec comes out warm, 10 to 12 min.
STIR arugula with garlic, lemon zest, juice and remaining 1/4 tsp salt in a medium bowl until combined.
SERVE half of roasted trout pieces with quinoa and top with arugula mixture (reserve remaining fillets for Trout Tacos).