Preheat oven to 400°. Lightly grease 2 baking sheets with cooking spray. In a large bowl, whisk eggs to combine. Add stuffing and milk, then stir to coat. Let sit until stuffing cubes are softened, about 10 minutes.

Meanwhile, in a large skillet over medium heat, melt butter. Add onions, celery, and a pinch of salt and pepper. Cook, stirring occasionally, until softened, 5 to 7 minutes. Let cool slightly.

Crumble sausage over stuffing mixture, then add onion mixture, parsley, and poultry seasoning; season with salt and pepper. Mix until just combined.

Roll stuffing mixture into 1 1/2 balls (about 2 Tbsp. each) and arrange on prepared sheets.

Bake bites, rotating sheets top to bottom halfway through, until deeply browned and cooked through, about 20 minutes.