1. Start by cooking the lentils, since they take the longest. Place lentils in a small pot, cover with at least 2-inches of water, and bring to a boil. Once boiling, reduce heat to medium-low and let cook for 20-30 minutes, or until lentils are tender. When lentils are finished cooking, they will be drained and added to the filling mixture.

2. Next, Peel and dice the potatoes, and place them in a medium-sized pot. Cover with water, then bring to a boil. Once boiling, cook for 10 minutes or until tender.

3. Peel and quarter the garlic cloves, and add them to the boiling water as soon as the potatoes are finished cooking. Drain the potatoes/garlic and place back in the pan (with the heat off). Roughly chop the spinach, and add it to the potatoes with all remaining ingredients, and mash to combine. Taste and add more salt/pepper as needed. Set aside while you make the filling.

4. Once potatoes are done (and lentils continue cooking), make the filling: thinly slice the mushrooms, and dice the carrot and onion into small pieces. Heat a large skillet over medium-high heat, and add the mushrooms, carrots, onion, oil, salt, and pepper. Sautee for about 10 minutes, stirring occasionally, until mushrooms are browned.

5. While mushrooms cook, finely dice the herbs and garlic. Measure out the vegetable broth, and stir the cornstarch into it, making sure there are no lumps.

6.Add herbs and garlic to the mushroom mixture and stir to combine, then immediately pour in broth/cornstarch mixture, soy sauce, and mustard. Increase heat to high, and stir until the mixture has thickened. Turn off the heat and add the drained lentils as soon as they're finished cooking.

7. Turn the oven on to BROIL. Pour filling into a large casserole dish and dollop with potatoes. Use a spoon to gently spread out the potatoes into an even layer. Top with an extra pinch of salt and pepper and a drizzle of olive oil. Place the casserole dish onto a sheet pan (in case it bubbles over) and broil for 5-8 minutes, or until potatoes are slightly golden and filling is bubbling.

8. You can serve this immediately, though it will stay hot for a solid 25-30 minutes, since it's so thick. If you'd like to make it in advance, let the filling and potatoes cool completely before layering in your casserole dish. Then wrap with plastic and store in the fridge for up to 5 days. Bake at 350 degrees Fahrenheit for 30-40 minutes, or until bubbling.