Place bread in a food processor; pulse 2 times or until crumbly. Sprinkle crumbs on a baking sheet; bake at 350° for 5 minutes or until golden. (Breadcrumbs should measure 1 1/2 cups.) Set aside. Increase oven temperature to 375°.
Place potato in a Dutch oven; cover with water. Bring to a boil. Reduce heat; simmer 15 minutes or until tender. Drain.
Melt butter in Dutch oven over medium-high heat. Add onion and garlic; sauté for 10 minutes or until browned. Add milk, salt, and pepper; bring to a simmer over medium heat, stirring frequently. Reduce heat; simmer 3 minutes. Place onion mixture in a food processor in batches, and process until smooth. Add potato, and process until smooth. Add 2 tablespoons chopped parsley and eggs; pulse 10 times or until combined.
Combine 1 1/4 cups breadcrumbs and 2 tablespoons chopped parsley; spread breadcrumb mixture into bottom of a 10-inch springform pan coated with cooking spray. Spread potato mixture over breadcrumb mixture; sprinkle with 1/4 cup breadcrumbs. Bake at 375° for 45 minutes or until golden brown. Run a knife around outside edge; cool 10 minutes. Garnish with parsley sprigs, if desired.