Preheat oven to 425 degrees F.

Cut sweet potatoes into 2 1/2-in.-long wedges.

Cut red onion into 1/2-in.-thick wedges.

On a large rimmed baking sheet, toss sweet potatoes and red onion with 1 tablespoon olive oil, 1/2 teaspoon salt, and 1/2 teaspoon pepper.

Roast for 20 minutes.

Halve the mushrooms.

Scatter the mushrooms on the pan and drizzle with 1 tablespoon olive oil.

Continue roasting until the vegetables are golden brown and tender, 8 to 10 minutes.

Season the sirloin steak with 1/2 teaspoon salt and 1/2 teaspoon pepper.

Heat the remaining tablespoon olive oil in a large skillet over medium-high heat.

Cook steak to desired doneness, 3 to 4 minutes per side for medium-rare.

Transfer steak to plates and cover with foil.

Add white wine to the pan and cook, scraping up any brown bits, for 30 seconds.

Add heavy cream and Dijon mustard and simmer until slightly thickened, 1 to 2 minutes.

Season the sauce with a pinch of salt and 1/2 teaspoon pepper.

Stir in chopped fresh tarragon.

Spoon sauce over the steaks.

Serve with the roasted vegetables.