Wash and dry all produce. Adjust broiler rack so that it is 3-4 inches from flame and preheat broiler to high. (TIP: Alternatively, preheat oven to 500 degrees.) Place potatoes in a medium pot with a pinch of salt and enough water to cover by 1 inch. Cover pot and bring to a boil. Cook until easily pierced by a knife, about 12 minutes, then drain.

Cut zucchini in half crosswise, then cut each half lengthwise into four pieces (you should have eight thick wedges). Toss on one side of a lightly oiled baking sheet with a large drizzle of oil. Season with salt and pepper. Cut sausages in half diagonally into wedge-shaped pieces. Arrange on other side of sheet.

Broil zucchini and sausages until browned on top, about 10 minutes. Once browned, flip zucchini, then add tomatoes to sheet. Return sheet to broiler and continue broiling until sausages are cooked through, zucchini is tender, and tomatoes are starting to burst, 2-3 minutes more.

In a small, microwave-safe bowl, stir together 1/4 cup water and demi-glace. Add thyme sprigs. Microwave on high until warmed through, about 1 minute. Stir to combine. Season with salt and pepper. Meanwhile, trim, then thinly slice scallions, keeping greens and whites separate.

After draining potatoes, return empty pot to stove over low heat. Add 2 TBSP butter and scallion whites. Toss until butter melts and scallion whites are slightly softened, about 1 minute. Add potatoes to pot, then mash with a potato masher or fork until smooth. Season with salt and pepper.

Divide potatoes between plates, then top with veggies and sausages. Remove thyme sprigs from gravy, then drizzle gravy over everything. Sprinkle with scallion greens and serve.