Tear the large spinach leaves into pieces. Fill a large bowl with cold water and wash the spinach. Repeat 2 or 3 times until the spinach is thoroughly cleaned. Drain and squeeze out excess liquid if necessary.
Heat a large skillet. Melt the butter, then add the olive oil, garlic, and chopped shallot. Cook until soft, about 3 minutes. Add the spinach by the handful and cook, stirring, until spinach is wilted.
Continue to add spinach and stir until all the spinach is in the pan. Add more olive oil as necessary. When all the spinach is wilted and cooked through, season with salt and pepper and toss again. Serve warm.