Madeira contributes a special flavor; add it right before serving.
Combine split peas and water in soup kettle. Bring to a boil, reduce heat, and simmer 2 minutes. Remove from heat and let stand 1 hour. Add vegetables, bay leaf, and thyme. Simmer, partially covered, 2-½ hours or until peas are soft. Press mixture through food mill. Return to kettle. Add knockwurst and additional water if necessary. Simmer 30 minutes. Season with salt, pepper, and Madeira. Garnish with croutons.