Heat the oven and place the crumbled keto bread on a baking sheet. Drizzle it with olive oil and bake for about 5 minutes, until the crumbs are lightly browned and feel dry rather than soft. The pieces should look a shade deeper in color and rattle lightly when the pan is moved.

Warm olive oil in a skillet, then cook the chopped celery, leeks, garlic, Italian blend seasoning, sage, salt, pepper, and optional celery seasoning for 1 to 2 minutes. Stop when the vegetables smell fragrant and the leeks turn translucent without developing dark edges.

Whisk the egg with the melted butter and chicken broth in a bowl, then stir in the warm vegetable mixture. The liquid should look evenly blended before it is added to the bread.

Fold the toasted bread crumbs into the seasoned liquid until coated, then transfer the mixture to a baking dish. Keep the crumbs loosely arranged so the casserole retains an airy texture.

Cover the dish with foil and bake at 350°F for 10 minutes, then remove the foil and bake for 5 minutes more. The edges should gently sizzle and the surface should turn golden and lightly crisp.

Serve the stuffing while the top is still crisp and the center is warm. Let it stand briefly if needed, until the egg-broth binder feels settled when scooped.