Butter an oven safe casserole dish. I like using a 9x9 square dish.
Place bacon lardons in a medium pan and cook over medium heat, stirring occasionally, until fat is rendered and bacon in cooked through. Remove bacon and set aside, leaving fat in the pan.
Sauté the onion, mushrooms and garlic in the bacon fat until just softened about 3 minutes.
Add kale to the pan and stir to combine. This remaining heat from the pan with soften the kale just enough. Add bacon back to pan, then remove pan from heat and let cool.
In a large bowl, whisk eggs, cream and a pinch of salt and pepper.
Combine bread cubes, sautéed vegetables, egg mixture, parmesan cheese and tarragon.
Place evenly into prepared baking dish and bake at 350 degrees for about 40 minutes or until eggs are firm and bread is toasty.