Pre-heat oven to 190°C.
Melt butter in a saucepan over medium-high heat, add onion. Fry, stirring often, 5 min, or until onion just starts to brown. Cover with a lid, reduce heat to very low, fry further 10mins, or until onion starts to caramelize.
Add stock, simmer further 5 min. Set aside.
Meanwhile, thinly slice zucchini and tomato.
Spread onion over the base of a tart dish, approx. 22cm in diameter.
Starting on the outside edge, layer zucchini and tomato slices alternately in a circular pattern Try saving the bigger slices for outside and smaller ones for the inner circles. Continue until onion is covered.
Brush vegetables lightly with oil and sprinkle with black pepper.
Bake 30 min. Serve hot.