Wash turnips, mushrooms, and carrots. Cut into 1/2" pieces. Heat oven to 425°F. Combine vegetables, toss with olive oil, and divide into batches between several baking sheets.
Roast in oven until browned and cook through - this can take between 10 and 20 minutes. Set aside.
Cut the root off of the leek(s) and remove the dark green leaves, keeping only the very light-green and white parts. Take off the outer layer. Cut the leek(s) into a small dice (about 1/4” pieces). Soak the leek pieces in a bowl of water for 5 minutes, then lift them out (don’t pour them out) and place them in a colander to drain.
Cut the celery into a small dice, and mince the garlic. Mince the herbs.
Heat 2 Tablespoons olive oil in a 2 quart pot over medium-low heat, and add the leeks, celery, and garlic. Sweat the veggies for 5-8 minutes until translucent – keep the heat low so they don’t color. Add the herbs, black pepper, and tomato paste, and cook for 3-4 minutes, stirring constantly so the tomato paste doesn’t stick and burn.
Add 4 Tablespoons olive oil to the pot, and then the whole wheat flour. Stir the flour and continue to cook for 4-5 minutes, stirring constantly. Slowly add the vegetable stock while stirring, starting with about 2 cups. Turn the heat up to medium-high and bring to a simmer, then add more stock if necessary to bring the sauce to a thick stew-like consistency. Remove from the heat.
Mix together the chickpeas, mushrooms, turnips, carrots, and filling ‘base’. You may not need all of the filling base – add a little at a time until you get to the desired consistency. It should be rich and fairly thick – not soupy. If needed, season to taste with kosher salt.
Heat oven to 350 degrees Fahrenheit. Wrap each yam/sweet potato in aluminum foil, place on a baking sheet, and roast in the oven until completely soft, 50 minutes. Remove from the oven and allow to cool.
Scoop out all of the flesh from the sweet potatoes/yams and put in a mixing bowl with the olive oil and black pepper. Mash with a big fork or potato masher. Whip the egg yolks with a little kosher salt until smooth, and add to the bowl. Mix until everything is combined.
Heat an oven to 350 degrees Fahrenheit. Spread the filling in the bottom of a 9×11” casserole dish. Spread the mashed sweet potatoes/yams evenly on top. Bake in the oven for 30 minutes, or until the top is lightly browned.
