Heat oil in a heavy bottomed skillet on medium-high heat. Saute the leeks with a pinch of salt and pepper, until they are translucent. Raise the heat to high and add the sausage and cook for 5-8 minutes or until browned.

Add in the kale as it will fit. Reduce the heat to medium, tossing and adding in more kale as there is room, until wilted but still bright green, 1-3 minutes.

Place the bread in a buttered 9×13 baking dish and top with the cooked sausage and kale mixture.

Whisk the eggs in a large bowl. Whisk in the milk, mustard, 1 tsp. salt and 1/2 tsp. pepper. Stir in the cheese. Pour over the bread mixture; cover and refrigerate for 1 hour or up to 2 days in advance.

Heat oven to 350 degrees. (If the strata is refrigerated overnight, remove from the refrigerator 40 minutes before baking). Bake until golden brown and set around the edges, but still wobbling in the center, 40-45 minutes. Let cool for 15 minutes before cutting. Serve warm or at room temperature.