Pulse the mushrooms in a food processor until they reach a ground meat like consistency. Transfer to a cheesecloth and ring out as much liquid as possible. Set aside.
Drain and rinse the lentils and squeeze out any excess moisture. In the food processor, puree half the lentils.
Heat a tablespoon of olive oil over medium low heat. Add in the onion and cook until lightly browned and caramelized, about 30 minutes. You can also save time and just saute them for 2 minutes before adding the other ingredients but caramelizing adds a lot of flavour.
Add the garlic, squeezed out mushrooms, the grated and well squeezed apples, maple, tamari, sage, rosemary, and thyme and season with a generous pinch of salt and pepper. Cook until any excess liquid evaporates and the mushrooms are browned.
Transfer to a bowl along with the lentils, pureed lentils, and oat flour and season with additional salt and pepper.
Form into a greased whoopie pie mold. Spritz with olive oil in an oil atomizer and bake for 17-20 minutes or until golden brown.