Cook pasta in a 4- to 6-quart pot of boiling salted water , uncovered, stirring occasionally, until al dente, then drain well in a coarse-mesh sieve. Transfer to a bowl and keep warm, covered. Meanwhile, heat oil and butter in a 12-inch heavy skillet over moderately high heat until hot but not smoking, then sauté garlic, red pepper flakes, and scallions, stirring occasionally, until garlic is golden, about 2 minutes. Add spinach and cook, stirring, until heated through, about 4 minutes. Toss pasta with spinach mixture and feta until combined. Season with salt and pepper.