Preheat oven to 375F/190C.
Start by dicing the onions or cutting them into quarter rings. Warm the olive oil and ½tbsp butter in a skillet/frying pan over a medium heat then add the onions. You want to cook them in a relatively thin layer, keeping them moist and avoiding them burning. Stir them now and again and reduce the heat if they are browning too quickly. They should brown gradually over around 20 minutes or more. Towards the end, add a little balsamic vinegar to add a little flavor and help the caramelization.
While the onions are cooking, lightly grease two small tartlet pans (2 ¾-3in diameter).
Make the bread into breadcrumbs (I use the food processor) and melt the remaining 2 ½tbsp butter.
Add the breadcrumbs to the butter and stir then divide the crumbs between the two pans and press the crumbs down and gently up the sides. Place on a baking sheet then bake the crusts for around 10-12mins until just starting to brown.
While the bases are cooking, make the filling by mixing together the goats cheese, egg and yogurt along with a couple grinds of salt and pepper until smooth (I used food processor but a blender or by hand will also work).
Once the bases are lightly brown, divide the onions into 4 approximately equal piles and spread ¼ into the base of each tartlet. Then spoon the cheese mixture on top and put back in the oven to cook for around 10-12minutes until just starting to brown and mixture is set.
While they are cooking, mix together the salad by tossing the arugula or other leaves, grapes and fennel in a little oil and vinegar.
Once they are ready, leave the tartlets to rest a minute before removing from the pans.
Top with the remaining onions and serve while still warm with the salad.