Beef Medallions with Spring Mashed Potatoes

INGREDIENTS

• 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