Heat the oven to 375 degrees and set a rack in the center.
Prepare the sweet potatoes: On a baking sheet, toss the sweet potatoes with the olive oil and a sprinkle of salt and pepper. Spread into an even layer and roast, stirring occasionally, until golden and tender, 15 to 25 minutes.
1 lbs Sweet potatoes, 3/4" cubes, 2 Tbsp Olive oil
While the sweet potatoes cook, prepare the lentils: Add the lentils to a medium pot and cover with about 6 cups of water. Salt the water generously and bring the mixture to a boil. Turn the heat down to a simmer and cook the lentils until just tender, 15 to 25 minutes. Drain the lentils then return them to the pot. Cover to keep warm.
1 cup Green or black lentils
Prepare the vinaigrette: Melt the butter. Cook the butter for a few minutes, stirring occasionally and scraping the milk solids off the bottom and sides of the pan as needed, until the milk solids turn golden brown and smell toasty.
4 Tbsp Butter, melted
Add the sage to a small bowl. Pour the hot browned butter and all of the toasty bits over the sage; it will crackle and foam a bit. Let the mixture sit for 1 minute to let the foam subside, then whisk in the olive oil, followed by the red wine vinegar and maple syrup. Season with salt and pepper to taste.
1 Tbsp Fresh sage leaves, chopped, 1 Tbsp Olive oil, 2 Tbsp Red wine vinegar, 1 tsp Maple syrup
Add the cooked sweet potatoes to the pot with the warm lentils. Pour the dressing over the top and stir gently to combine. Add the parsley, season with more salt and pepper, if desired, and toss to combine.
1/2 cup Fresh parsley, chopped
Transfer the mixture to a serving dish and sprinkle with goat cheese, if using. Serve warm.
1/2 cup Feta cheese