I love caramelized onions. For my mom's birthday tea, I wanted a vegetarian tea sandwich for the savory course. I found a log of Brie that worked perfectly. You can always just buy a wheel and slice. All the components can be made a head and then warmed in the oven just before serving. I have used the jam to top chicken and as a spread for hamburgers. —mcms1974
In a large frying pan, warm the oil and then add onion slices. Sprinkle with a nice pinch of salt and gently cook over medium low heat until deeply brown. This will take about 20 minutes. Be patient this needs to be done low and slow.
Add the mustard powder, brown sugar, pepper, thyme and vinegar. Cook, stirring often until vinegar evaporates, about 5 minutes. Set aside until ready to assemble crostini.
Cut the baguette into 3/4 inch slices brush with olive oil. Toast at 350 degrees until golden. Slice the Brie into twelve pieces.
On each crostini spread onion jam, place a piece of Brie and sprinkle walnuts on top. Bake in the oven a few minutes until cheese is slightly melted but not runny. Serve immediately.