In a large pot, saute the leeks, carrots, celery, and garlic in the avocado oil with the thyme and bay leaf over medium-low heat, covered, until veggies are beginning to get tender (about 10 minutes). Stir occasionally.
Add the beans, salt, pepper, and water to the pot. Bring to a simmer over medium heat, then turn the heat down to medium low and simmer gently, partially covered, for 25 minutes.
While the cassoulet is simmering, preheat the oven to 350 degrees. Toss the bread crumbs with the olive oil, garlic powder, salt and pepper. Spread the mixture in an 8 by 8 inch baking pan and toast it in the oven for about 15 minutes, until golden and crisp. Stir once or twice during the 15 minutes.
When the cassoulet is finished cooking, taste it and add more salt and pepper if needed. Remove the bay leaf and fresh thyme. Sprinkle with bread crumbs just before serving.