How to Make It
Preheat oven to 450°F. Bring water and quinoa to a boil in a medium saucepan over high. Reduce heat to medium-low; cover, and simmer until tender, about 12 minutes. Remove from heat; let stand 3 minutes. Fluff with a fork, and pour onto a large baking sheet to cool, about 20 minutes. (You should have about 5 cups cooked quinoa). Transfer to a large bowl.
Toss together sweet potatoes, 2 tablespoons of the oil, 2 teaspoons of the chili powder, and 1 teaspoon of the cumin on a large rimmed baking sheet. Arrange in an even layer, and bake until tender and browned, 15 to 20 minutes, stirring after 10 minutes. Let cool 20 minutes. Transfer to bowl with quinoa.
While quinoa cools, heat a large cast-iron skillet or grill pan over medium-high. Rub steak with 1 teaspoon of the salt, and remaining 2 teaspoons cumin and 1 teaspoon chili powder; spray steak and scallions with cooking spray. Add steak to skillet, and cook until browned and a thermometer inserted in thickest portion registers 125°F for medium-rare, 3 to 4 minutes per side, or until desired degree of doneness. Transfer to a cutting board, and let rest 5 to 10 minutes. Add scallions to skillet, and cook until charred, about 6 minutes, turning once. Transfer to cutting board with steak. Coarsely chop scallions, and cut steak into 3/4-inch chunks. Transfer to bowl with quinoa. Add beans, corn, radishes, cilantro, and pepitas; stir to combine.
Whisk together honey, zest, lime juice, and remaining 4 tablespoons oil and 1 1/2 teaspoons salt in a small bowl; drizzle over quinoa mixture, and toss to coat. Garnish with chile slices, if desired. Serve immediately, or cover and chill until ready to serve.