1 pkg. turkey kielbasa, halved lengthwise and sliced
1 tablespoon vegetable oil
2 cups chopped sweet onions
1 tablespoon minced garlic
1 1/2 cups light heavy cream
1/2 cup low-sodium beef broth
1/2 cup spicy brown mustard
2 tablespoons chopped fresh chives
2 teaspoons chopped fresh thyme
2 teaspoon kosher salt
1 teaspoon black pepper
8 oz. shredded smoked Gruyere cheese, divided
Stale bread on a baking sheet until completely dry, overnight.
Preheat oven to 375 degrees. Coat a 9x13-inch baking dish with nonstick spray.
Brown kielbasa in oil in a large skillet over high heat, 3-5 minutes. Drain kielbasa on a paper-towel-lined plate; reserved drippings. Reduce heat to medium; add onions and garlic to drippings and cook 1-2 minutes; transfer to bowl.
Whisk together eggs, cream, broth, mustard, chives, thyme, salt, and pepper in large bowl. Add bread; toss to coat. Add kielbasa, onion mixture and half of the Gruyere; transfer to prepared baking dish, cover with plastic wrap and let sit in the refrigerator overnight.
Remove wrap and sprinkle remaining half of Gruyere over casserole; bake until an instant-read thermometer inserted in center reaches 155 degrees; about 45 minutes. Let casserole rest 15 minutes before serving. 8 servings.