Preheat oven to 425°. In a 5- to 6-qt. pot, heat 3 tbsp. oil over medium-high heat. Add chile, celery, carrots, and chopped onion and cook, stirring often, until vegetables soften, about 5 minutes. Add 2 tsp. garam masala and cook, stirring, about 30 seconds. Add thyme and 1 tsp. mustard; cook 1 minute.
Stir in lentils, 3 cups broth, and 1/2 tsp. salt. Cover and bring to a boil, then reduce heat and simmer until lentils are just tender, about 40 minutes, checking occasionally and adding more broth if needed so they're covered in liquid.
Meanwhile, in a 9- by 13-in. baking dish, whisk together honey; 1/2 tsp. salt; and remaining 4 tbsp. oil, 2 1/2 tsp. garam masala, and 1 tsp. mustard. Add squash and toss until well coated.
Roast squash 15 minutes. Stir in onion chunks and brussels sprouts and cook until vegetables are tender when pierced with the tip of a knife, 15 to 20 more minutes.
Drain any excess liquid from lentils and discard chile. Stir in orange juice and transfer lentils to a shallow bowl. Spoon roasted vegetables on top, serve with crème fraîche, and season to taste with more salt.
*Find black beluga and French green lentils at well-stocked grocery stores and online.