2 large or 3 medium sweet potatoes (1½–2 lb. total)
1 cup lentils, preferably black beluga or French green
6 Tbsp. extra-virgin olive oil, divided
4 Tbsp. unseasoned rice vinegar, divided
2 Tbsp. pure maple syrup
1 Tbsp. soy sauce
1 Tbsp. white miso
1 tsp. toasted sesame oil
1 tsp. mild red pepper flakes, plus more for servingCoarsely chopped toasted pistachios (for serving)
Preheat oven to 425°. Roast sweet potatoes on a rimmed baking sheet until tender, 45–55 minutes. Let cool 10 minutes.
Cook lentils in a medium pot of boiling salted water until tender but not falling apart, 20–30 minutes. Drain; let cool 10 minutes.
Toss lentils in a medium bowl with 4 Tbsp. olive oil and 2 Tbsp. vinegar. Season with salt.
Trim scallions and cut crosswise into 3″ lengths. Slice each piece into very thin matchsticks. Place in a small bowl and pour in cold water to cover; set aside.
Whisk maple syrup, soy sauce, miso, sesame oil, 1 tsp. red pepper flakes, remaining 2 Tbsp. vinegar, and ¼ cup water in a small bowl. Set glaze aside.
Heat remaining 2 Tbsp. olive oil in a large nonstick skillet over medium-high. Slice or tear sweet potatoes into 2″ pieces; cook, turning, until deeply browned on several sides, 5–7 minutes. Remove from heat and add reserved glaze (it may spatter a bit). Set over medium heat; cook, spooning glaze over, until thick enough to coat a spoon, about 1 minute.
Scrape sweet potatoes and glaze onto a platter. Drain scallions; pat dry. Toss two thirds into lentils; spoon around potatoes. Top with nuts, remaining scallions, and more red pepper flakes.
Do ahead: Sweet potatoes can be roasted and lentils cooked and dressed 5 days ahead. Let cool. Cover and chill separately.