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.

In a pot, bring the water and rice to a boil over medium heat. Cover with a lid, and adjust the heat to medium-low. Simmer for 20 minutes, or until the rice is tender. Stir the lemon juice, parsley, lemon zest, and salt into the rice until evenly combined. Taste and add more salt, if preferred.

Combine lettuce, tomatoes, and red onions in a bowl, then divide it between 4 plates or containers. Then, divide the rice and lentil balls between each plate. Garnish with a lemon wedge and serve.