Rinse lentils in a few changes of water until washed. Soak in cold water for 2 hours or until slightly hydrated.
Once soaked, transfer lentils into a medium saucepot and cover well with water. Add bay leaf and bring to a boil. Cook steadily for 25-30 minutes or until the lentils are soft but not falling apart. Remove from heat, and drain. Remove bay leaf and set lentils aside.
In a large saucepan heat olive oil and add onion. Sweat for about 1 minute and add mushrooms, salt and garlic. Add walnuts (if using) along with the remaining ingredients. Sautee for another 1-2 minutes or until the mushrooms have slightly dried and the herbs and spices smell toasty.
Add lentils, oats and the water. Bring to a boil, lower heat and simmer steadily for 15-20 minutes. Add more water if necessary; the mixture should be slightly loose but not runny. Check for seasoning and add more salt if needed. (I added another ¼ teaspoon salt.) Remove from heat and cool to room temperature.
Preheat oven to 400℉/204℃. Use a 7 or 8 inch pie plate or springform pan. Roll ¾ of the dough out onto a well floured surface large enough to fit the pan with overhang. Transfer into the pie plate and press to fit well.
Add cooled filling and roll out the remaining dough to form the top crust. Brush some water on the outer edge of crust before topping with the other piece of dough. The water will act as a "glue." Pinch the edges together well and form into any shape you like. Brush crust liberally with milk all over and make a hole in the middle of the top crust. Bake for 15 minutes, then reduce the oven temperature to 375℉/190℃ and bake for another 40-45 minutes. The crust should be golden brown.
Allow the tourtière to rest for 10-15 minutes before slicing. Serve with ketchup and enjoy!