Mixed Vegetable Potage

Name: Mixed Vegetable Potage
Ingredients: 1 3/4 pounds spinach, stemmed and washed well; 2 pounds potatoes, peeled, halved lengthwise, and cut across into slices; 12 medium carrots, peeled and sliced; 2 medium onions, cut into chunks; 10 cups basic chicken stock, or commercial low sodium chicken broth; 3 cloves garlic, smashed and peeled; Kosher salt, to taste; Freshly ground black pepper, to taste; 3 tablespoons unsalted butter