Step 1

Preheat oven to 350°. Heat ¼ cup oil in a large Dutch oven over medium-high until smoking. Add mushrooms and cook, undisturbed, until golden brown underneath, about 5 minutes. Turn mushrooms over and cook, undisturbed, until browned on the other side, about 5 minutes. Season mushrooms with ½ tsp. salt; transfer to a plate.

Step 2

Add 3 Tbsp. oil to same pot and add sausage, arranging in a single layer. Reduce heat to medium and cook, undisturbed, until browned underneath, about 3 minutes. Turn sausage over and cook on second side until browned, about 3 minutes. Add chopped fennel bulb, onion, carrots, celery, sliced garlic, and 1 tsp. salt and cook, stirring often until vegetables are softened but have not taken on any color, 10–12 minutes. Add thyme, paprika, and red pepper flakes and cook, stirring, 1 minute. Add tomato paste and cook, stirring occasionally, until paste darkens in color and begins to stick to the bottom of pot, about 1 minute.

Step 3

Stir mushrooms, lentils, and 1 tsp. salt into sausage mixture and add 8 cups water. Increase heat to medium-high and bring to a boil. Reduce heat to low, cover pot, leaving lid slightly askew, and continue to cook, stirring a few times, until most of the liquid has been absorbed and stew is thick and lentils are barely al dente, 25–30 minutes.

Step 4

Meanwhile, pulse bread in a food processor until coarse crumbs form (the largest ones should be about ½"). Transfer breadcrumbs to a large rimmed baking sheet and arrange in an even layer. Drizzle with remaining ¼ cup oil and toss to evenly coat. Bake until deep golden brown, 25–30 minutes. Remove from oven and finely grate remaining 3 garlic cloves over breadcrumbs; toss to incorporate. Add fennel fronds (you want to have about ½ cup) and toss again to combine.

Step 5

Scatter half of breadcrumbs evenly over stew. Transfer pot to oven and cook stew, uncovered, until some of the breadcrumbs have cooked into the stew and stew is bubbling around the edges, 13–15 minutes.

Step 6

Scatter remaining breadcrumbs over stew and cook until stew is very thick, 10–15 minutes.

Step 7

Do Ahead: Kielbassoulet can be made 3 days ahead. Let cool; cover and chill. Reheat, uncovered, in a 350° oven.