In a large sauté pan over medium heat, add the oil. Add the sausages and cook until browned on all sides, about 5 minutes. Transfer to a plate to cool then slice into 1-inch pieces.
Return the pan to the heat and cook the onions and garlic until beginning to brown, about 8 minutes. Stir in the herbs, bay leaf, parsley, and tomatoes and bring to a boil. Reduce the heat and simmer until slightly thickened, about 5 minutes. Add sausage and beans and simmer until heated through, about 5 minutes.
While the cassoulet simmers, make the topping: In a small sauté pan over medium heat, add the oil and heat until almost smoking. Add the breadcrumbs and sauté until lightly browned. Stir in garlic, parsley, and sauté 1 minute. Season to taste with salt and pepper.
To serve, remove and discard the bay leaf. Transfer the bean mixture to a 1-quart serving dish and sprinkle with breadcrumb mixture. Serve immediately.