Step 1
In a large Dutch oven or heavy pot over medium-high heat, heat oil. Add onion, fennel, and sausage and cook, breaking up sausage into large pieces, until vegetables are softened and sausage is browned, about 8 minutes.
Step 2
Stir in buckwheat, parsley, and garlic and cook, stirring, until garlic is fragrant, about 1 minute. Add broth and bring to a boil over high heat, scraping up any browned bits. Cover, reduce heat to medium, and cook, stirring occasionally, until buckwheat is just starting to become tender, about 20 minutes.
Step 3
Uncover and continue to cook, stirring occasionally, until liquid is reduced and buckwheat is plump and tender (about 10-15 min more).
Step 4
Add spinach and tomatoes and stir until spinach is wilted. Remove from heat and stir in half of parsley leaves and half of Parmesan; season with salt and pepper.
Step 5
Divide farro mixture among bowls. Top with remaining parsley leaves and Parmesan.