Preheat Instant Pot to saute setting. Season chicken thighs with salt. Add 2 tablespoons grapeseed oil to pot, swirl to coat, and add chicken thighs in two batches so as not to overcrowd. Saute until golden, flipping once, about 4 minutes per side, transferring them to a large plate as they finish. Add remaining 1 tablespoon grapeseed oil and red onion. Season with salt and saute, stirring occasionally, until softened and onions turn golden in spots, about 5 minutes more. Add orange juice and zest, rice wine vinegar, soy sauce, chicken broth, garlic, ginger, honey, and red pepper flakes. Stir to combine, scraping bottom of pan to deglaze. (You can also do this in a skillet over high heat; cooking time will be reduced.).
Place chicken thighs (and onion mixture, if you used a skillet on the stovetop) in pot, nestling them into as even a layer as possible.
Cover and cook on high pressure for 18 minutes. Release the pressure manually. Return Instant Pot to saute setting.
Transfer thighs to a plate and cover to keep warm. Place cornstarch in a medium bowl. Gradually ladle approximately ¼ cup of cooking liquid into cornstarch, whisking constantly until smooth. Pour cornstarch mixture back into the pot. Cook, whisking, until mixture has come to a boil and thickened slightly. Add snap peas and continue cooking, stirring occasionally, until snap peas are crisp-tender, about 2-3 minutes. Serve immediately with pan juices and rice, garnished with sesame seeds and scallions, if desired.