Wash and pat dry the leg of lamb. Rub with the lemon juice and pour melted butter or olive oil over. Season generously with salt, pepper, oregano and thyme. Place in a large baking pan and pour wine, 1 cup water, the tomato puree, minced onion, garlic and cinnamon around meat (if you have an earthenware casserole - even better!).
Give the sauce a stir and roast, turning meat over occasionally when you see a crust forming (to brown all sides) for 1 to 1 1/2 hours (note: Greeks like their lamb well-done, if you prefer a shorter cooking time, cook to your personal preference).
In a large saucepan, bring plenty of water to the boil, salt generously and boil the orzo pasta for 8 minutes. Remove and drain - retain a cup of the cooking water. Add the pasta to the lamb casserole and give everything a stir to distribute orzo throughout the pan.
Increase heat to 400-degrees and continue to bake another 15-20 minutes, or until pasta is fully cooked and has absorbed most of the liquid in the pan (it should still be fairly 'juicy' - not dry). If the pasta is too dry, stir in enough of the pasta cooking water you've retained to keep it saucy.
Have lots of freshly-grated parmesan or kefalotiri cheese to sprinkle over pasta at table.