Bagged spinach or bunch spinach?
Sometimes I do buy the bagged spinach, because it's so much easier. But bunch spinach is better, really. It's a little bit cheaper, but mainly it's better because fewer people handle it, and so it is less likely to have nasty germs than the bagged spinach is. Also, if you get your spinach at the farmer's market, it's not going to come in a bag.
How to make sauteed spinach:
Take one bunch of spinach and pull off the leaves. Compost the stems. In a medium-size frying pan, heat 1/8 cup of olive oil over medium-high heat. Dump in the spinach leaves. If there's too much to fit all at once, put in half and wait for it to wilt a little, then add the other half when there's room.
Use salad tongs to lift and toss the spinach in the frying pan so the top leaves cook and the bottom leaves don't burn. When the leaves are pretty wilted, in about two minutes, take the pan off the heat. Add the juice of half a lemon, and it's done.
Vegetarian or vegan
Sauteed spinach is completely vegan, no worries! It's also vegan if you eat it raw, just the leaves in a bowl as a salad with a little vinegar.