In a large bowl, whisk together 1 cup lemonade, 1/3 cup olive oil, minced garlic, 1 tablespoon Dijon, and a generous pinch of kosher salt and black pepper.
Add the chicken to the marinade, turning to coat. Cover and refrigerate for at least 30 minutes or up to overnight.
While the chicken marinates, cook 3 cups couscous according to package directions; set aside.
Preheat the grill or grill pan to high heat. Toss broccoli florets and sliced bell peppers with a little olive oil and season with salt and pepper.
Use tongs to lift the chicken from the marinade, letting excess drip off, and place directly on the grill. Grill 5–6 minutes per side, until an instant-read thermometer registers 165°F (74°C).
Grill the vegetables alongside the chicken, turning occasionally, until tender and lightly charred, about 6–10 minutes total.
Pour the remaining marinade through a strainer into a saucepan. Bring to a boil over medium heat, then reduce heat and simmer until reduced by half.
Remove chicken from the grill and let rest 5–10 minutes. Slice the chicken.
Toss the grilled vegetables with the cooked couscous and fold in the crumbled feta and any herbs or microgreens if using.
Arrange couscous and vegetables in bowls, top with sliced chicken and pickled onions, then drizzle the warm reduced marinade over the bowls. Serve immediately.