Preheat oven to 425°F. Coat a (24-cavity) mini-muffin pan with cooking spray.
Arrange bread on a baking pan in a single layer. Bake until toasted and dry, about 10 minutes. Remove from oven and place in a large bowl. Reduce oven temperature to 350°F.
Heat butter in a large skillet over medium-high until foamy, about 2 minutes. Add sausage; cook about 4 minutes, stirring occasionally, until lightly browned. Add apple, onion, garlic, and sage; cook 3-4 minutes, stirring occasionally, until apple and onion are slightly softened. Remove from heat and cool 5 minutes. Transfer to bowl with bread.
Add chicken stock, parsley, salt, pepper, nutmeg and eggs to bowl with bread mixture; toss gently until moistened. Divide mixture among muffin cups. Bake at 350°F until set, about 20 minutes. Cool in pan 10 minutes. Use a small offset spatula to carefully remove from pans; serve warm.