Put the spinach in a large microwave-safe bowl.

In a small heavy skillet, heat the oil over medium-low heat.

Add the garlic, onions, and pumpkin seeds to the skillet, and cook for about 5 minutes, reducing the heat if necessary to avoid browning (and burning seeds!), until the onions are tender and the garlic is fragrant.

Pour the onion mixture plus the balsamic or other sweet vinegar over the spinach and toss gently.

Steam the spinach in the microwave, partially covered, for about 2 minutes until it is just wilted. Season it with salt and pepper, to taste, and serve it immediately (if you prefer you can wilt the spinach in the skillet instead).