Preheat your oven to 180°C (355°F).
Grease and line an 8 x 8-inch (20 x 20 cm) baking tin with parchment paper, leaving some overhang.
Trim and grate the zucchini using the large holes of a box grater.
Squeeze out as much moisture as possible from the grated zucchini and weigh 400 g.
In a large bowl, whisk the eggs until smooth.
Add the flour and baking powder, whisking until no lumps remain.
Stir in the prepared zucchini, onion, roasted red pepper, feta, olive oil, oregano, Parmesan, salt, and pepper.
Mix until everything is evenly combined.
Pour the mixture into the prepared baking tin and spread evenly.
Bake the Mediterranean Zucchini Slice for 45–50 minutes until golden and set in the center.
Test with a skewer; it should come out clean.
Allow the slice to rest for 15 minutes before removing from the tin.
Slice and serve warm or cold with flaky sea salt if desired.