Cook garlic in half the oil, then add courgettes, season, cover, and gently cook 35–40 mins, uncovering for the last 10 mins. Set aside.
Preheat oven to 180°C fan (200°C conventional). Butter and dust a deep dish with polenta.
Make a roux with butter and flour; gradually whisk in milk until smooth. Off heat, mix in yolks, goat's cheese, most of the dill, and courgettes.
Whisk egg whites to soft peaks. Fold a little into the mix, then gently combine the rest.
Pour into dish, smooth top, run a knife around edge, sprinkle remaining dill.
Bake 20–25 mins until risen and golden. Serve immediately.
