Slice the onions as finely as possible. Heat a large heavy bottomed pan and add a little oil and the butter then start to fry the onions very gently. Season them at the outset with salt and fresh milled pepper. Continue frying them until they are soft and lightly caramelised, stirring in the thyme leaves after five minutes or so. Do not rush this stage. Once done, spread out on a tray or plate to cool.
While waiting, bring a large pan of water to the boil and then salt it generously. Plunge the greens into the water and blanch for 10 seconds then drain at once and chill quickly under cold running water. Once chilled, drain and squeeze out excess water. Set aside on a plate for now.
To make the breadcrumbs for the topping place the bread in a food processor and whiz until finely ground to the right consistency. (If your bread is on the fresh side for doing this, place the bread in the oven for a few minutes to dry it out before processing it). Set aside until later.
Spread the cooked onions on the base of an oven proof gratin dish and arrange the spring greens all over to cover. Slice or tear the goat cheese and scatter this over the greens. (All these stages can be done in advance and refrigerated like this until needed).
To finish, sprinkle the breadcrumbs over the top and place in a warm oven at 180°C / 350°F / Gas Mark 4 for 10 minutes until warmed through, the goat cheese should be just melting and the breadcrumbs pale golden brown.
