Heat avocado (or canola) oil in a large flat-bottom wok or cast-iron skillet over high heat. Add shrimp and scallions; cook, stirring, until the shrimp turn pink, about 2 minutes. Add spinach and cook until wilted, about 30 seconds. Whisk the sauce and add it to the pan. Cook, stirring constantly, until the sauce thickens, about 1 minute.