Bring the sorghum and 3 cups water to a boil in a medium saucepan over high heat. Reduce the heat, cover and simmer until the sorghum is tender and the water is absorbed, 50 to 60 minutes; season with salt and pepper.
Meanwhile, put the lentils in a medium saucepan with enough water to cover by about 3 inches; bring to a boil. Reduce the heat to low and simmer, uncovered, until tender, 25 to 30 minutes; drain.
Heat the oil in a medium skillet over medium heat. Add the celery and onion and cook until softened, about 8 minutes. Stir in the garlic; cook for 1 minute. Stir in the cooked lentils; season with some salt and pepper. Add the lentil mixture, spinach and herbs to the sorghum; toss to coat. Serve.