If mushrooms pieces are large, snip or break into tiny bits. If mushrooms feel sandy, rinse them.
Combine mushrooms, lentils, water, salt, and thyme in a large, heavy saucepan. Cover and bring to a boil. Simmer until almost tender, 10 to 15 minutes. Stir in Heritage® O's. Add a little water if mixture seems dry. Cover and continue cooking until lentils are soft and Heritage O's have softened, about 5 minutes more.
While lentils are cooking, heat oil in a large skillet. Add onions and cook over medium heat until lightly browned, stirring occasionally, about 5 minutes.
Stir in garlic and cook another minute. Stir in vinegar and cook just until it evaporates, about 20 seconds. Stir onion mixture, parsley, and 3 tablespoons sun-dried tomatoes into lentils. Season to taste with salt and pepper. Let cool for 15 minutes.
Line bottom and sides of standard loaf pan with a one large piece of plastic wrap. Arrange a few whole parsley leaves and remaining sun-dried tomatoes decoratively on bottom. Pour in lentil mixture and smooth off top. Cool to room temperature. Cover and chill until firm, about 1 hour, or up to 5 days. (Flavour improves over time.)
To serve: Turn out onto a plate and gently peel off plastic wrap. Cut into 8 to 10 slices.