Leek and Pearl Onion Gratin

For her birthday, I pulled together two of my wife's favorites into one creamy gratin dish - leeks and perl onions.  Turned out to be a great gift!
2 large leeks, trimmed, cleaned, cut in half lengthwise and widthwise
1 Tbsp butter plus 1 Tbsp olive oil
1 lb frozen peeled pearl onions
1 Tbsp butter plus 1 Tbsp olive oil
1 cup half and half
1/4 tsp fresh grated nutmeg

Topping mixture:
1/4 cup panko bread crumbs
1 Tbsp olive oil
1 clove garlic, minced
1/4 tsp dried dill weed
1/4 tsp salt
dash freshly ground black pepper

1 cup cubed Swiss or Gruyere cheese

Preheat oven to 375F. Melt butter in the oil in a large non-stick skillet. Add leek, cut side down and slowly brown over medium heat, about 5 minutes.

Watch carefully so as not to burn. Gently turn leek over and add 1/4 cup water.

Cover and braise the leek for about 10 minutes, until very soft to the tip of a knife. Transfer leek to a baking dish.

Melt the next batch of butter and oil in the skillet and tip pearl onions into to skillet.

Brown the pearl onion, shaking often, until they soften and brown. This will take about 10-15 minutes over medium-low heat.

While the onions cook, toss together the topping mixture in a small bowl.

Once the onions are nicely caramelized and soft, add the half and half and bring to a gentle boil. 

Lower heat to a simmer and and reduce liquid by half.

Scatter the cheese over the leeks.

Tip onions into casserole on top of leeks. Top with panko toping mix.

Bake in oven for about 20-30 minutes, until the casserole in browned and bubbly. Serves 4-6 as a side dish.