Leek and Spinach Tart

1 sheet frozen puff pastry, thawed (or your basic pate brisee for an 11" tart)
2 tbsp butter
1-3 large leeks (white and pale green parts only), sliced
1 tsp dried thyme
 
1 ½ – 2 cups of spinach leaves, coarsely chopped
1 1/4 cup of whole milk
3 large eggs
2 large egg yolks
1 tsp salt
1/4 tsp ground black pepper
 
Roll out the pastry on a floured work surface to a 12" square. Transfer to a tart pan or a 9" glass pie dish (this is what I used). Fold overhang under and crimp edges. Chill until ready to use.
 
Melt the butter in a large skillet over medium-low heat. Add leeks and thyme. Season with salt and pepper and sauté, stirring often, until leeks are very tender but haven't begun to brown yet (7-10 min). Add spinach and sauté for a couple of minutes until it's wilted. Remove from heat and set aside until ready to use.
 
Preheat oven to 425F. In a large bowl, whisk milk, eggs, yolks, salt and pepper. Mix in the cooled leeks and spinach. Pour filling into the crust.
 
Bake for 15 min on 425F. Reduce heat to 350F and bake for 10-15 min longer until filling is puffed and completely set in the center. I find that it's best at room temperature with a little bit of grated cheese on top.