Wash and dry all produce. Preheat oven to 425 degrees. Halve potatoes, quarter larger ones so all are equal-sized. Halve, peel, and slice onion into 1/2-inch wedges. Place mustard in a large bowl with 4 tsp olive oil. Stir to combine.

Add potatoes and onions to mustard, and stir to coat. Season with salt and pepper. Spread on a baking sheet, and roast until golden brown, about 25-30 minutes.

Place chicken on another baking sheet, and season on both sides with salt, pepper, and dried thyme. Roast until firm and juices run clear when pierced with a knife, about 20 minutes.

Meanwhile, mince garlic. Cut lemon into wedges.

Cook the spinach: With about 5 minutes lef on potatoes, heat 1 TBSP butter in a large pan over medium-high heat. Add garlic and cook until fragrant, about 1 minute. Add spinach and season generously with salt and pepper. Toss until wilted, 2-3 minutes.

Finish and plate: Plate chicken with potatoes and onions on the side. Serve with spinach and a lemon wedge for squeezing over. Enjoy!