Preheat your oven to 350°F (175°C). Spread the bread cubes on a baking sheet and toast them in the oven for 10–15 minutes, or until they are golden and dry, stirring halfway through.

In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add diced onions or shallots and sauté for about 3 minutes until they become translucent. Stir in the sliced mushrooms, seasoning with salt and pepper, then cook for 10–15 minutes, until the mushrooms are nicely browned. Finally, add the chopped kale and cook until wilted, which should take about 4–6 minutes.

In a large mixing bowl, whisk together 6 eggs and 2 cups of half and half until well blended. Add 1 cup of shredded Gruyère cheese along with a pinch of salt and pepper to taste. Gently fold in the toasted bread cubes and the sautéed vegetable mixture, ensuring everything is evenly coated in the custard.

Transfer the combined mixture to a lidded dish or a greased 9x13-inch baking dish. Cover it tightly and refrigerate for at least 2 hours; overnight is even better.

When ready to bake, preheat your oven to 350°F (175°C) if it has cooled down. Remove the lid and place the dish in the oven, baking the strata uncovered for 45–60 minutes or until the top is golden brown and the center is set.

After baking, allow the Kale and Mushroom Strata to cool for about 10–15 minutes before slicing. Slice into squares and serve warm.