In a saucepan of cold water, bring the beets and 1/4 teaspoon salt to a boil. Reduce heat, and simmer until tender, about 30 minutes. Drain and cool. Peel beets, and cut into 1/4-inch pieces; set aside.
Combine 1 cup water, lentils, stock, garlic, thyme, and 1/4 teaspoon salt in a saucepan; bring to a boil. Reduce heat; simmer until tender, about 30 minutes. Drain; remove thyme stems and garlic. Add oil, 1 tablespoon balsamic vinegar, and 1/8 teaspoon pepper; toss, and keep warm. (If preparing in advance, warm over low heat before assembling dish).
Heat oven to 375 degrees. Combine coriander and 1/4 teaspoon each salt and pepper; rub over salmon flesh. Spray a cast-iron skillet with vegetable-oil spray, and place over medium heat. When heated, add salmon, flesh side down, and cook until browned, about 3 minutes. Turn fillets, add shallots, and place skillet in oven. Cook about 4 minutes, or until cooked through.
Meanwhile, place spinach in a metal steamer basket over boiling water; season with 1/4 teaspoon salt and 1/8 teaspoon pepper. Cover, and steam until just wilted. Remove and discard salmon skin; keep salmon warm. Place skillet over medium-high heat, and add wine, scraping pan with a wooden spoon. Reduce by half, and add orange juice, mustard, and remaining tablespoon balsamic vinegar. Add beets, and cook until heated through. Adjust seasoning, if desired. Arrange lentils and spinach on plates; place a salmon fillet on top. Spoon beets and sauce over, and serve.