Cut zucchini into 1/2s or 1/3s, cored on one end (the other end is left intact) and remove filing carefully with corer so that a not-too-thick wall of zucchini remains to contain the filling, keep 1/2 the zucchini flesh removed from the coring, minced (not the shells- but the actual part of the zucchini that was removed, the other half makes a lovely omelette with a bit of parmesan for another meal!).
You can prepare your zucchini from the previous day and keep, wrapped, in the refrigerator- this actually makes a lot less work on the day of preparation as it's the coring of the zucchini that takes up the most time.
Mix the meats, onion, garlic, rice, parsley and zucchini in a bowl and season to taste with salt and pepper.
Stuff this mixture into the opening of the one end of each zucchini piece and make sure the filling goes all the way into the vegetable.
Level opening neatly so that no filling comes out.
Layer the filled zucchini pieces in a big pot and add enough boiling water to just cover.
Put in the stock cubes and the butter and cover.
Simmer for 45 minutes to 1 hour (knife tip should slip into a piece easily), and take off heat.
Carefully remove the zucchini pieces with a slotted spoon and place on a reheatable serving dish.
Strain the cooking liquid left in the pot to remove any pieces of zucchini or grains of rice that may be floating around.
In a large bowl, whisk the eggs with the cornstarch and lemon juice (add the juice of 1 lemon at this point).
Slowly ladle in (literally the thinnest of streams) your hot cooking liquid, whisking the eggs all the while so they don't curdle.
Keep doing this till nearly all the cooking liquid has been used and then pour your egg mixture and any cooking liquid left back into the pot and heat on a low flame until the sauce has thickened.
Taste, if it needs more lemon, add up to 1/2 more.
This is what is called an'Avgolemono Sauce' (Egg-lemon Sauce)- it is delicious.
When ready to serve, put 2-3 piping hot pieces of the stuffed zucchini on each plate and nap with the sauce.
Serve with feta cheese on the side (you won't believe how good a nibble of feta tastes with each bite of this!), good, crusty bread (you want to get all the sauce), and wine.