Melt 2 tablespoons butter in large nonstick skillet over medium heat.
Add leeks, onions and thyme; sauté until leeks begin to soften, about 4 minutes.
Cover and let rest. Transfer to medium bowl; cool.
Preheat oven to 300°F.
Mix potatoes, flour, egg, salt and pepper into leek mixture in medium bowl.
Melt 6 tablespoons of butter and line a sheet pan with parchment paper; paint the parchment with melted butter.
Put potato mixture on sheet pan and spread even.
Paint another piece of parchment paper with butter and put on top of potato mix; cover with another sheet pan for weight.
Bake in oven for 90 minutes or till the mix develops a light brown, crispy surface.
Before serving, re-warm in 250°F oven about 10 minutes. Serve hot.