Spread challah cubes in single layer in a large baking pan. Bake, stirring occasionally, until golden brown, about 10 minutes. Transfer to a large bowl. In a large, heavy frying pan over medium high heat, melt butter. When hot, add the onion and celery and cook, stirring occasionally until golden brown, about 20-25 minutes. Transfer onion and celery to bread cubes. Add stock( my mom always added warm stock), a little at a time until the bread is completely moistened, but not mushy. Add ground thyme, parsley and salt and pepper. I do not add the egg until I have the seasonings just right. Mix in egg and stir to combine.
Fill the cavities of the turkey with stuffing and truss.
Butter 6 large or 12 standard muffin tins. Use an ice cream scoop to fill and mound up the stuffing in the muffin tins. Bake until set and crisp on top; 15-20 mins. Everyone gets their own individual crispy Stuffing Muffin!