Place the bacon in a medium saucepan over medium heat. Cook the bacon until crispy, about 7 to 8 minutes. Remove the bacon from the pan with a slotted spoon, drain, and reserve.

Brown the sausages in the pan until golden brown, about 2 to 3 minutes. Transfer the sausages from the pan to a platter and keep warm.

Add the onions, fennel, and carrots to the saucepan and cook, stirring occasionally, for 3 minutes. Add the garlic and bay leaf and saute until fragrant, about 30 seconds. Add the lentils and stir to coat in the fat, about 1 minute. Stir in the thyme, tomatoes, and chicken stock, and bring to a boil. Reduce to a gentle simmer and cook 50 to 55 minutes, or until the lentils are tender and have soaked up most of the liquid. Add the spinach and cook until just wilted. Remove and discard the bay leaf and thyme sprigs.

Slice the sausages on a diagonal about 1 inch thick.

Serve the lentils with the sliced sausages on top with the mustard and crusty French bread.