© Marlene Cornelis/Urban Cottage Life.com

Pre-heat oven to 350℉. Brush a 2-quart casserole dish (or combination of casserole dishes) lightly with oil.

Celeriac Mash

• one medium celeriac (celery root) about 1 lb 5 oz
• 1 medium baking potato (about half the weight of the celeriac)
• 1 tbsp olive oil
• plain almond milk or vegetable stock, as required
• kosher salt
• freshly ground pepper

Lentil & Mushroom Filling

• 1 cup large green lentils, uncooked and rinsed (makes just over 2 cups once cooked)
• 1/2 lb portobello mushrooms
• 1/2 cup lentil plus celeriac cooking liquid
• 2 tbsp olive oil
• 1 medium onion, finely chopped
• 1 large clove garlic, minced
• 1 generous tbsp tomato paste
• 1 tsp dried thyme
• 1/2 tsp kosher salt
• 1/4 to 1/2 tsp freshly ground pepper
• pinch red pepper flakes
• 1/4 cup finely chopped flat leaf parsley
• 1 tbsp balsamic vinegar

Peel the celeriac and potato, wash and cut into relatively equal chunks. Cover with water in a medium cooking pot, bring to a boil and then simmer until the vegetables are soft. Drain, reserving a half cup of the cooking liquid. Add the oil to the celeriac and potatoes, and mash roughly. Add a couple pinches of salt and a good grinding of pepper according to your taste. Pour in a quarter cup or so of the almond milk or stock and mash. Continue to add liquid in small increments, mashing and stirring vigorously after each addition until a creamy, somewhat spreadable consistency is achieved. Note, I find the celeriac doesn’t mash as smoothly as the potatoes, so there will be some texture. Set the mash aside.

Place the lentils in a medium-sized pot and add water to about an inch above the lentils. Cover and bring to a boil, then turn the heat to simmer just the lentils are cooked through but still have a bit of bite to them (al dente). This will take 15 to 20 minutes — check frequently to ensure they don’t overcook. Drain the lentils, reserving about a half cup of the cooking liquid (mix it with the celeriac cooking liquid).

Wipe the mushrooms with a damp paper towel or cloth. Once clean, trim the stems and cut into strips about 1 cm wide. Set aside.

In a large skillet, heat the oil over medium high heat. Add the onion and cook, stirring, for a few minutes until the onion softens. Stir in the garlic, then the tomato paste. Stir the tomato paste until well incorporated; this allows the flavour to mellow a bit. Toss the mushrooms into the skillet and stir well. As they cook, they will begin to release their juices. Once this has happened, add the cooked lentils, along with the salt, pepper, thyme and red chili flakes. Stir well.

Add about 1/4 cup of the lentil/celeriac liquid. After another 5 minutes, add another 1/4 cup or as much as is needed so the mixture is neither dry nor runny. Stir in the parsley and the balsamic vinegar. Taste to check for seasonings, and adjust the salt, pepper and/or vinegar if necessary.

To assemble, spread the lentil and mushroom mixture in the bottom of the prepared casserole dish. Dollop the celeriac/potato mash all over the top, then use a spatula to spread. If you like, you can make a pattern in the top with the tines of a fork. Lightly brush a drizzle of olive oil around the top of the casserole. I like to grind some more pepper over the top as well.

Bake at 350 ℉ for 30 to 40 minutes, or until heated through and the mash is starting to brown and go crispy at the edges.