Preheat your oven to 400°F/200°C
Heat 2 tablespoon of olive oil in a large heavy-bottomed pot over medium-high heat on the stove top.
1 tablespoon extra virgin olive oil
Add the onions and sauté until they've softened slightly.
1 large yellow onion
Add the garlic and spices and cook for a few minutes more.
2 cloves garlic, 1 teaspoon salt, ½ teaspoon dried thyme, ½ teaspoon freshly cracked black pepper
Then add the carrots and celery and sauté 5 minutes more, until the vegetables are slightly tender.
4 medium stalks celery, 2 large carrots
Add the lentils, canned tomatoes, and vegetable broth. Bring to a boil, then reduce the heat to low, cover, and simmer until the lentils are tender — about 20 minutes.
1 ½ cups lentils, 14 oz can crushed tomatoes, 3 cups vegetable broth
When the lentils are tender, stir in the frozen peas.
1 cup frozen peas
Spoon the lentil filling into a casserole dish, then top with mashed potatoes.
4 cups mashed potatoes
Bake in a pre-heated oven at 200°C / 400°F for 20-30 minutes, until the potatoes are golden and the filling is bubbling around the edges.