Cut off the tops and bottoms of the zucchinis. Rinse them well and then cut them in half about 2,5 inch / 7cm each piece.
Using an apple corer, remove most of the insides of the zucchini. Creating 1 cm / 0.4 inch thick cylinders.
Prepare the filling in a bowl by mixing together the ground beef, red onion, spring onion, parsley, dill, rice, and the 3 tablespoons of olive oil. Season the mixture with kosher salt and freshly ground pepper. Knead the mixture for 2 minutes.
Start filling the zucchinis by pressing the ground beef mixture into the hole with your finger. Press on both openings to make sure there aren't any air gaps left, inside the zucchini.
Place the stuffed zucchinis tightly together, inside a cooking pot or stockpot.
Pour in enough water to cover, and heat over medium-high heat.
Once the water starts to boil, skim off the foam that will form on the surface. Then reduce heat to medium-low, and simmer covered for about 1 hour.
Once the water has reduced to almost half, turn of the heat.
Start making the sauce. In a small saucepan, whisk together the flour and olive oil to create a thick flour paste. Heat mixture over high heat and cook for 2 minutes. Then using a ladle, start adding spoonfuls of the water from the pot with the zucchinis, into the flour mixture. Keep whisking and adding spoonfuls, until you have incorporated at least ⅔ of the water. In a mixing bowl whisk the eggs until slightly foamy. Take a spoonful of the sauce you've just made and very slowly (threadlike) pour it into the eggs while you whisk the eggs constantly. Keep doing this until all the sauce is into the eggs. Pour in the lemon juice + zest as well. Then transfer everything back to the pot with the zucchinis.NOTE: Place a wet sponge cloth under the bowl with the eggs so it will stay in place while you whisk the eggs.
Shake the pot for the sauce to spread evenly. Turn heat to low, taste the sauce and add extra salt or pepper if needed. Cook for 2-3 minutes for the sauce to combine with the zucchinis.
Remove from heat and ideally let the food stand for 30 minutes before serving. Then serve with a bit of extra fresh dill on top, freshly ground pepper and a few drops of lemon if you like.