Bring a medium pot of salted water to a boil. Add lentils and cook 15 to 20 minutes, until al dente. Drain well and set aside.
In a large, deep skillet, heat olive oil over medium heat. Add rosemary leaves and cook, stirring frequently, 1 to 2 minutes, until slightly crispy. Remove rosemary from pan and place on a paper towel-lined plate. Set aside, leaving the infused oil in the pan.
To the pan of reserved oil, add carrots, celery, and onion. Cook, stirring frequently, 4 to 5 minutes. Add garlic and continue cooking an additional 1 to 2 minutes.
Stir in the tomato paste, lentils, red wine, and 1 cup of water. Bring to a boil, then reduce heat to low. Simmer for 15 to 20 minutes, until the sauce has thickened slightly.
While the sauce simmers, cook fettuccine according to package directions. Drain and set aside.
Crumble the reserved crispy rosemary back into the sauce. Season with salt and black pepper to taste. Serve the bolognese over fettuccine.