Cook rice: Preheat broiler with top rack 6 inches from heat source. Rinse rice under running water until water runs clear. Transfer to a small saucepan, add 11⁄4 cups water and a pinch salt, and bring to a boil. Reduce heat to low, cover, and cook until rice is tender and water is absorbed, about 17 minutes. Keep covered until ready to use.
Prep ingredients: Meanwhile, trim ends from onion, then halve, peel, and slice. Trim ends from zucchini and cut into 1⁄2-inch thick rounds. Halve lemon lengthwise, and cut one half into wedges (save rest for own use). Pick and chop leaves from half of the thyme stems (save rest for own use). Peel and finely chop 2 large cloves garlic .
Prep chicken & veggies: On baking sheet, toss onion, zucchini, lemon wedges, and half the chopped thyme with 1 tablespoon oil, 1 teaspoon salt, and several grinds pepper; spread in an even layer. Pat chicken dry and pound to an even thickness if necessary. Rub with oil and season with 1⁄2 teaspoon salt and a few grinds pepper.
Broil vegetables: Broil vegetables on top rack until browned, stirring halfway through, 10–12 minutes (watch closely). Whisk together 1 tablespoon of the mustard (save rest for own use) and 2 tablespoons water in a small bowl.
Cook chicken & make sauce: Heat 1 tablespoon oil in a large skillet over medium-high. Add chicken; cook until golden, 3 minutes per side. Remove chicken from skillet; take skillet off heat. Add garlic, butter, and 1 tablespoon oil; cook, stirring, until golden, 2 minutes. Add Dijon mixture to skillet and whisk quickly to combine, scraping up any brown bits in the pan.
Finish & serve: Pour garlic–Dijon sauce over chicken and veggies, and sprinkle with the remaining chopped thyme. Gently toss together on baking sheet to combine; season to taste with salt and pepper. Divide rice between plates and serve chicken and veggies over top with any pan sauce. Enjoy!