Heat the oven to 220°C (425°F) with a large rimmed sheet pan inside. Take the venison out of the refrigerator 30 minutes before cooking.
Trim the silverskin: slide the tip of a thin knife under it and run the blade along the length of the meat, angled upward, until the silvery strip comes away. Pat the venison dry.
Toss the squash with 2 tablespoons oil and a generous pinch of salt. Carefully spread on the hot sheet pan and roast for 15 minutes, until the edges begin to brown.
Rub the venison with the remaining oil, salt, pepper, and thyme.
Stir the squash and push it to the sides. Set the venison in the center. Roast for 12 to 15 minutes, until a thermometer in the thickest part reads 54°C (130°F) and the surface is browned and firm to the touch, with a little give.
Move the venison to a cutting board, tent loosely with foil, and rest for 8 minutes. Return the squash to the oven for 5 more minutes if needed, until deeply golden at the edges.
Melt the butter in a small skillet over medium heat. Cook the shallot for 2 minutes, until soft and translucent. Add the broth and simmer for 3 minutes, until reduced by half.
Add the cream and mustard. Simmer gently for 3 minutes, until the sauce coats the back of a spoon. Remove from heat and stir in the tarragon, lemon juice, salt, and pepper.
Slice the venison across the grain into 1/2-inch (1.3cm) slices. Serve over the squash with the sauce spooned on top.