Set a colander or fine-mesh sieve over a large bowl. Pour butter beans from can into colander, letting juices drain into bowl. Set beans and bean juice aside.
In a large skillet over medium heat, melt butter. Add shallot and ½ tsp. salt and cook, stirring frequently, until softened, about 3 minutes. Add garlic and cook, stirring, until fragrant, about 1 minute more.
Add cream, water, lemon zest, lemon juice, ½ tsp. salt, reserved beans, and ¼ cup bean juice to pan. Bring to a simmer over medium-high heat and cook, stirring occasionally, until liquid is reduced by half, 4 to 5 minutes.
Fold in spinach and cook, stirring frequently, until wilted, 1 to 2 minutes; season with pepper. If sauce is too thick, add 1 to 2 Tbsp. water to reach desired consistency; season with more salt, if needed.
