Potato Drum (Timbale or Timpano) - Testing

http://www.grouprecipes.com/1038/tater-timbale.html

1. I made this for Isabelle's b-day dinner. I have mixed feelings about it. I have no idea what size of ramekins to use. There were loads of potato left over, lots of left over apple... left over everything, except maybe for the custard, but then I think I overfilled the ramekins because lots of the custard spilled out and burned onto the cookie sheet they were baking on.
2. In completely rethinking this recipe, I really dislike the idea of the individual ramekins. I'm thinking of the gorgeous timbale in Big Night and so I'm changing the instructions to bake it in a springform pan. In executing the recipe, the amount of ingredients is perfect! It did not cook through though as everything was a little underdone. The question is whether the problem is the amount of time or the temperature. 
3. Well, I'm not getting the results I want but because of no fault of the recipe. I forgot to put in a third of the onion, I only remembered the cheese once I'd already built half the dish up, and the springform pan leaked most of the custard. The only innovation that I'll be able to assess is the melted butter I brushed on top - will it help the potato to brown? Oh, and I increased the temperature to 375F. BUT I have another idea. In Big Night the Timpano is made in a pot that needs to be turned over, sort of like a tarte tatin. And I've seen some timpano recipes where a bundt cake mold is used. Ergo, I shall try the same. And it'll help bake the middle more evenly since, well, there is no middle. ALSO I wonder about adding some ground fennel instead of fennel bulb for flavor. FINALLY, I want to work out the internal temperature I need to make sure the middle is cooked properly. I'm starting with what is recommended for baked potato at 210F.
4. I removed the Fennel and Sweet potato, but I wonder about replacing them with celeriac or even Jerusalem Artichoke? Future File.

2lbs 8oz potatoes, sliced very thin

1 apple, peeled and cored, sliced thin  

2 Tbsps olive oil

2 medium onions med(12oz/340gr), sliced 

2 cloves garlic, minced  

Chèvre goat cheese 90 g 

Sharp cheddar 90 g 

2 eggs 

⅓ cup  milk 

⅓ cup heavy cream 

A dash of nutmeg 

1 tsp salt 

Pepper 

butter to brush on top

1. Butter a plain bundt pan. Maybe cut out a parchment paper ring for the bottom?
2. Preheat the oven to 375°F.
3. Bring a pot of salted water to boil.
4. The potato and sweet potato cook together for 5 to 7 minutes.
5. Cook the onion in olive oil with the garlic, until soft (about 10 minutes).
6. In a bowl, beat together the eggs and milk, and season with salt and pepper. Add the nutmeg.
7. Line slices of potatoes around the edge of the pan and place some at the bottom. Keep them tight.
8. Continue with a layer of onions and a layer apple slices and cheese.
9. Continue layering until you run out of room, finishing with a layer of potato.
10. Melt a little butter in a pan and brush the top to help the potatoes become golden.
11. Pour in the egg batter, making sure that it goes down all the way to the bottom.
12. Cook in the oven for 45 to 50 minutes. The internal temperature should be 210F. 
13. Check whether the top potatoes are cooked and wait a few minutes when taking out before un-molding on a plate by turning the bundt pan over.