How to Make It
In a large heavy pot, brown the sausage over medium heat, turning occasionally, until golden, about 6 minutes. Transfer the sausage to paper towels to drain, keep the fat in the pot.
Stir the onions, garlic, and 1/2 teaspoon each salt and pepper into the fat in the pot and cook, stirring occasionally, until golden, 6 to 8 minutes. Stir in the carrots, celery, stock, and split peas and bring to a simmer. Cook, stirring occasionally, until the split peas are very soft and falling apart, about 1 1/2 hours. In a blender, puree the soup in batches until smooth. Stir in the reserved sausage and cook until the sausage is warmed through, about 2 minutes. Season the soup with salt and pepper to taste and serve.