Heat the oven to 375°F and line a rimmed sheet pan with parchment. Soak breadcrumbs in milk for 5 minutes.
Stir raw eggs, onion, garlic, oregano, salt and pepper into the crumbs, then fold in beef. Keep hard-boiled eggs and spinach separate.
Pat about two-thirds of the meat into a 10-by-6-inch rectangle on the pan, forming a shallow channel down its center. Spread spinach in the channel and place peeled cooked eggs end to end on top, leaving meat exposed around every edge.
Cover with remaining meat and pinch all seams closed, forming a loaf about 10 inches long and 3 inches high. Mix passata and oil and brush over the top.
Bake for 60 to 70 minutes until the meat beside and beneath the egg filling reaches 160°F, checking several locations without measuring an egg alone.
Rest for 15 minutes and slice into 8 portions through the egg centers.