Trufflemania: It's the culinary world's Beatlemania, except it never ends and there are fewer screaming girls involved. This demi sauce with truffle oil let's you taste a bit of the hype, without paying those high truffle prices. It's the perfect sauce for the sirloin steaks (and we won't tell if you dip the fingerling potatoes in it as well).
Halve fingerling potatoes lengthwise. Toss potatoes on prepared baking sheet with 2 tsp. olive oil, ½ tsp. salt, and a pinch of pepper. Spread into a single layer and bake 20 minutes. Potatoes will finish cooking in a later step. While potatoes bake, prepare ingredients.
Prepare the Ingredients
Mince garlic. Peel and slice shallot into ¼" rounds. Mince parsley stems and leaves. Rinse peas under warm water if still frozen. Rinse steaks, pat dry, and season both sides with ¼ tsp. salt and a pinch of pepper.
Sear the Steaks
Heat 2 tsp. olive oil in a medium non-stick pan over medium-high heat. Place steaks in hot pan and cook until steaks are browned and reach a minimum internal temperature of 145 degrees, 4-5 minutes per side. Remove steaks to a plate and tent with foil. Rest at least 5 minutes. Reserve pan; no need to wipe clean. While steaks rest, finish potatoes.
Finish the Potatoes
After potatoes have baked 20 minutes, add peas, garlic, and shallots to baking sheet and toss. Bake until potatoes are golden brown and tender, 6-8 minutes. Remove from oven and garnish potatoes with parsley. While potatoes bake, make sauce.
Make the Sauce
Return pan used to sear steaks to medium-high heat. Add demi-glace, Dijon, and ¼ cup water to hot pan. Cook, stirring occasionally, until sauce is thick enough to coat back of a spoon and simmering, 2-3 minutes. Remove pan from burner and stir in half the truffle oil. Taste, and add more if desired.
Plate the Dish
Place a serving of peas and potatoes on a plate. Add sauce to plate. Place steak in sauce.