Wash and dry all produce. Adjust rack to upper positions and preheat oven to 450 degrees. Wash and dry all produce. Cut potatoes into 1/2-inch cubes. Halve, peel, and dice onions into 1/2-inch pieces. Mince or grate garlic. Halve lemon. Halve zucchini lengthwise, then cut crosswise into 1/2-inch-thick half-moons. Roughly chop cilantro.

Toss potatoes on a baking sheet with a large drizzle of olive oil and a pinch of salt and pepper. Roast on top rack, tossing halfway through, until tender and crisped, about 25 minutes.

Meanwhile, pat pork dry with paper towels; season all over with salt and pepper. Heat a large drizzle of olive oil in a large pan over medium-high heat. Add pork and cook, turning occasionally, until browned all over, 4-8 minutes total. Turn off heat.

Toss onions and zucchini on a second baking sheet with a large drizzle of olive oil and a pinch of salt and pepper. Transfer pork to same baking sheet. Roast on middle rack until veggies are tender and pork is cooked through, 12-15 minutes. Transfer pork to a cutting board and let rest 5 minutes.

Heat a large drizzle of olive oil in pan used to cook pork over medium heat. Add garlic and cook until fragrant, 1-2 minutes. Add demi-glace and 3/4 cup water. Cook, scraping up any browned bits from bottom of pan, until slightly reduced, 3-4 minutes. Turn off heat. Stir in 2 TBSP butter until melted. Season with pepper and a squeeze of lemon juice.

Slice pork crosswise. Divide potatoes, veggies, and pork between plates. Top veggies with a squeeze of lemon juice (to taste). Drizzle pork with pan sauce. Garnish everything with cilantro.