Heat oil in a large skillet over medium-high heat. Add sausage and cook, breaking it up into large chunks with a spoon, until browned, 6 to 8 minutes. Remove with a slotted spoon to a 6-quart slow cooker; reserve skillet. Add wine to reserved skillet and cook, scraping up brown bits, until thickened, 30 seconds; add to slow cooker. Add onion, celery, garlic, stock, Italian seasoning, fennel seeds, bay leaf, and 1/2 teaspoon pepper to slow cooker and stir to combine. Cover and cook until vegetables are tender, on low for 5 to 6 hours or high for 2 to 3 hours. Stir in escarole and tortellini; cover and cook until escarole is wilted and tortellini is tender, 14 to 16 minutes on low or 4 to 6 minutes on high. Season with salt and pepper. Serve topped with Pesto Croutons. Make the Pesto Croutons: Preheat oven to 400 degrees F. Stir together pesto and olive oil in a bowl. Add ciabatta bread and toss to coat. Bake until toasted, 10 to 12 minutes.