Step 1Cut off and discard the root ends of the garlic cloves. Place the cloves in a small saucepan, and add enough oil to cover them by about 1 inch. None of the garlic cloves should be poking through the oil.
Step 2Place the saucepan on a diffuser over medium-low heat; alternatively, place the saucepan over low heat. The cloves should cook gently: Very small bubbles will come up through the oil, but the bubbles should not break the surface. Adjust the heat as necessary and move the pan to one side of the diffuser if it is cooking too quickly. Cook the garlic for about 40 minutes, stirring every 5 minutes or so, until the cloves are completely tender when pierced with the tip of a knife. Remove the saucepan from the heat and allow the garlic to cool in the oil. Refrigerate the garlic, submerged in the oil, for up to a month.
Step 1If using large-leaf spinach, remove the thick stems and wash the leaves. If necessary, wash the baby spinach.
Step 2Melt the butter in the largest skillet you have over medium-low heat. Add the shallots and saute gently for 1 minute. Add the garlic confit and only as much spinach as will fit in the skillet, sprinkle with one-half teaspoon salt and a pinch of pepper, or to taste, and use tongs to turn the spinach in the butter. As the spinach wilts, continue to add handfuls of spinach from time to time and season additionally as needed. Do not overcook: The spinach is done when it is wilted and tender but still bright green. Serve immediately.