Big Batch Chicken Thighs with Fennel and Scallions

8 garlic cloves, finely grated
1½ tsp. crushed red pepper flakes
2 Tbsp. honey
5 Tbsp. grapeseed or vegetable oil, divided
8 skin-on, bone-in chicken thighs
2½ tsp. kosher salt, plus more
3 large fennel bulbs sliced into ¾”-thick wedges (reserve stems and fronds for another use)
8 scallions, trimmed
1 lemon, halved
Toasted sesame oil (for drizzling)

Place racks in upper third and middle of oven; preheat to 425°. Whisk garlic, red pepper flakes, honey, and 2 Tbsp. grapeseed oil in a large bowl. Season chicken thighs with 2½ tsp. salt and toss to coat in marinade. Arrange, skin side up, on a parchment-lined rimmed baking sheet; let sit at room temperature while you prepare the vegetables.
Toss fennel wedges and scallions with remaining 3 Tbsp. grapeseed oil on another rimmed baking sheet. Season with salt, then arrange in a single layer. Roast vegetables on top rack and chicken on middle rack, tossing vegetables halfway through, until vegetables are softened and chicken is browned and cooked through, 35–45 minutes.
Divide scallions, two-thirds of the fennel, and 4 chicken thighs between plates. Drizzle with sesame oil and squeeze a lemon half over. Transfer remaining chicken thighs along with any juices from baking sheet to an airtight container; let cool, then cover and chill. Place remaining fennel and lemon half in a separate airtight container; let cool, then cover and chill.
Do Ahead: Chicken and vegetables can be roasted 3 days ahead. Keep chilled.