Preheat oven to 375 degrees and prepare a baking sheet with foil. Throughly rinse produce and pat dry. Peel and halve shallot. Slice into thin strips (julienne). Cut tomatoes into 1"-thick rounds (about 3 per tomato). Peel potatoes and cut into 1” dice.

Place a colander in the sink. Place potatoes in a small pot, cover with lightly salted water, and bring to a boil. Reduce to a simmer and cook until tender, about 14-16 minutes. Drain in colander and return to pot with half the butter, shredded cheddar cheese, and half the milk. Mash with potato masher or fork and add remaining milk 1 Tbsp. at a time until desired consistency is reached. Season to taste with salt and pepper. Remove to a bowl, cover and set aside, and rinse out pot to make peas.

While potatoes cook, heat a medium pan over medium-high heat. Add 2 tsp. olive oil and sausages to pan and brown on two sides, about 5 minutes total. Transfer to prepared baking sheet and roast for 15 minutes, or until a minimum internal temperature of 165 degrees is reached. Set aside.

Return pan used to sear sausages to high heat (no need to clean). Add tomato rounds to pan and sear 1-2 minutes on each side, or until slightly charred. Transfer to a plate and set aside. Add shallots to pan and cook until soft, about 2-3 minutes. Add demi-glace base and 1/2 cup water and stir to incorporate. Cook until thickened to a pan gravy consistency, about 2-3 minutes. Set aside.

Place peas, 2 Tbsp. water, and remaining butter in small pot used for potatoes over medium-high heat. Cook until warmed through, about 2-3 minutes. Season to taste with salt and pepper.

Place a mound of cheddar mash on a plate and top with shallot gravy. Arrange tomatoes on one side of mash and peas on opposite side. Arrange sausages on top of mash and serve.