Preheat the oven to 375ºF (190ºC, or gas mark 5), and line a baking sheet with parchment paper or a silicone baking mat.
Place the lentils, walnuts, dried mushrooms, tomato paste, parsley, salt, and pepper into a food processor equipped with an S-blade.
Pulse the mixture until it is broken down into smaller pieces, but is not yet a paste.
Fold the bread crumbs into the lentil mixture until combined.
Form twenty 2-tablespoon-sized (30 g) lentil balls with your hands, and place them on the baking sheet.
Bake for 10 minutes, then flip them over, baking for an additional 10 minutes. Cool on a rack for 7 to 10 minutes before removing them.