Preheat oven to 425°F
Cut cauliflower into quarters, lengthwise, and remove core. Cut into small pieces and place on baking tray.
Drizzle with 2 tbsp of olive oil, sprinkle with salt and pepper. Toss to mix.
Remove enough thyme leaves from stems to produce about 1 tsp of chopped thyme, set aside. Place remaining sprigs over cauliflower.
Roast for 15 minutes, stirring half way through. Remove from oven and discard thyme sprigs. Reduce oven temperature to 350°F.
While cauliflower is roasting prepare the leeks.
Remove the dark green ends of leeks, keeping only the white and light green parts. Cut off the root end. Slice in half lengthwise then slice across into approximately 1/2 cm (1/4") slices.
Place in large bowl and fill with cold water. Agitate leeks with your hand to release any dirt. Let stand in water for 1-2 minutes. Lift leeks from bowl and into a colander to drain.
Heat 1 tbsp olive oil in large cast iron skillet over medium heat. Saute leeks for approximately 5 minutes or until soft. Leave in pan but remove from heat and set aside while preparing the other ingredients.
Grate both cheeses and mix together in a bowl.
In a large bowl whisk together eggs, yogurt, salt and pepper. Add approximately 3/4 of the cheese mixture and stir
When cauliflower is done, add to skillet with leeks. Mix in chopped thyme. Pour over the egg mixture and stir gently to combine. Sprinkle with remaining cheese and bake for 50-60 minutes or until set in the center. If desired, broil for 2-3 minutes to brown the top. Serve warm or at room temperature.