Rub a splash of olive oil in a 9-inch / 23cm square baking dish (or equivalent). Alternatively, line it with parchment paper. Sprinkle the baking dish or parchment paper with lemon zest and set aside.

In a medium bowl, whisk the olive oil, mustard, salt, and pepper with a splash of the milk. Whisk in the rest of the milk and the eggs.

Put the bread seed in the prepared baking dish and top with the spinach and half of the feta. Gently toss with your hands—just enough that the spinach and cheese mixes with all those pieces of bread. Make sure the bread is relatively level in the baking dish. Very slowly drizzle the egg mixture over the bread and sprinkle with the remaining feta. Cover and refrigerate overnight.

The next morning, when you are ready to bake the strata, preheat the oven to 350°F / 175°C with a rack in the top third of the oven.

Bake the strata, uncovered, for 45 to 55 minutes, until the egg is set in the middle and the edges are browned. You need to cut into it a bit to be sure it is well cooked, particularly in the center. It can be a bit tricky to tell; err on the side of overdone versus underdone. I like to brown the top a bit more under the broiler (low setting) before removing the strata from the oven, but it's an optional extra step. Skip this step if you've lined your pan with parchment paper.

Serve warm, drizzled with a bit of olive oil and sprinkled with the fresh oregano.