1. Place potatoes in a large bowl.
2. Sauté onion in oil until golden.
3. Add sautéed onion and rest of ingredients to mashed potatoes and mix together well, and set aside.
4. While potatoes are cooking, you can start preparing the zucchini layer: Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius).
5. Finely grate zucchini in the food processor and drain well.
6. Place in a bowl and add remaining ingredients, mixing well until everything is well incorporated.
7. Grease a 6-muffin silicone pan.
8. Place a small amount of zucchini mixture on the bottom of each indentation, pressing down gently to form an even layer.
9. Fill the rest of the indentation with the potato mixture, and bake for 30 minutes.
10. Remove from oven and let sit for three minutes, so potato layer settles a bit.
11. Spray tops of muffins with cooking spray and arrange zucchini slices in a circle.
12. Brush with egg if desired.
13. Return to oven and bake for 10 more minutes.
14. Turn off oven and leave pan inside for five minutes. (If you prefer to skip the garnish, bake for 35–40 minutes or until ready.)
15. Remove from oven, and cool completely before popping out of pan.
16. You will have enough leftover mixture to make a 9-inch square Pyrex pan kugel as well, do the same as you would for the muffins and bake for 40–45 minutes.