Melt the butter in a large skillet over medium heat. Add the shallots, salt, and several grinds of pepper and cook for 4 minutes, or until softened. Stir in the garlic, followed by the flour.

Add the cream and reduce the heat to low. Simmer, stirring occasionally, for 5 to 10 minutes, or until the cream is thick enough to coat the back of a spoon. Add the cheese and stir until melted.

Add the spinach and cook, stirring frequently, until just wilted. The sauce will thin out slightly from the water released by the spinach. Cook for an additional 2 to 3 minutes to thicken the sauce, then stir in the lemon juice.

Season to taste and serve.