Roast beet: Heat oven to 425 degrees. Toss beet with 1 tsp oil in a baking dish and season with salt and pepper. Cover tightly with foil and roast until tender, 45 to 60 minutes, depending on size. When cool enough to handle, peel skin and slice.
Meanwhile, stew the lentils: Place lentils, shallot, garlic, bay leaf, and enough water to cover ingredients by 2 inches (about 5 cups) in a medium saucepan and bring to a boil. Reduce heat and simmer until lentils are tender, about 20 minutes. Drain liquid and return lentils to pan. Season with salt and pepper. Stir in vinegar and remaining 2 teaspoons of the oil.
Spread goat cheese on bread. Top one slice bread with beet slices, 1/4 cup lentils, and pea shoots. Top with second slice.