Beef Medallions with Spring Mashed Potatoes

1 1/2 lb. Yukon Gold potatoes, peeled and cut into 1-inch pieces

1 tsp. coarse kosher salt, plus more, to taste

3 Tbs. olive oil

1/2 head small savoy cabbage, cored and thinly sliced crosswise

Freshly ground pepper, to taste

2 Tbs. unsalted butter

1 1/4 lb. beef tri-tip roast, excess fat trimmed

2 large shallots, halved lengthwise and thinly sliced crosswise

1 Tbs. all-purpose flour

2 cups low-sodium beef broth

2 Tbs. dry vermouth

2 tsp. Dijon mustard

1 1/2 Tbs. minced fresh thyme, plus more for garnish

Put the potatoes in a large pot. Add water to cover by 2 inches and then add the 1 tsp. salt. Bring to a boil and cook until the potatoes are tender, about 10 minutes. Meanwhile, in a large nonstick fry pan over medium heat, warm 1 Tbs. of the olive oil. Add the cabbage and season with salt and pepper. Sauté until almost tender, about 8 minutes. Set aside.

Reserve 1 cup potato cooking liquid, then drain the potatoes. Return the potatoes to the pot and mash coarsely with a fork, thinning with the cooking liquid as needed. Mix in 1 Tbs. butter, the cabbage and a generous amount of pepper. Adjust the seasonings if needed. Cover to keep warm.

Cut the beef across the grain into 1/4-inch slices and then cut into 2-inch lengths. In a large fry pan over medium-high heat, warm 1 Tbs. of the olive oil. Working in batches, add the beef in a single layer and cook until just colored, about 30 seconds per side. Transfer to a plate and season with salt and pepper.

Add the remaining 1 Tbs. olive oil to the fry pan. Add the shallots, reduce the heat to medium, and stir until tender and golden brown, about 6 minutes. Add the flour and stir for 20 seconds. Gradually stir in the broth and vermouth. Simmer, stirring frequently, until the sauce thickens slightly, about 6 minutes. Stir in the mustard and the 1 1/2 Tbs. thyme. Remove the pan from the heat and stir in the remaining 1 Tbs. butter until melted. Adjust the seasonings if needed.

Transfer the beef and potatoes to a platter or divide among 4 warmed plates. Drizzle the sauce over the beef, sprinkle with thyme and serve immediately. Serves 4.