If you want to make ahead, you can assemble the onions and peppers in the baking dish a day ahead. You can roast the peppers and toast the baguette slices a few hours ahead of serving, and assemble the crostini an hour or two before serving.
This appetizer is best served at room temperature.
- 24 (1/4-inch thick) slices of baguette
- 2 small red onions, peeled, thinly sliced (2 to 3 cups)
- 3 tablespoons extra virgin olive oil
- 1 tablespoon balsamic vinegar
- 1/4 teaspoon salt
- 1 cup water or stock
- 12 mini (2- to 3-inch) red, yellow, and orange sweet peppers
- 1 tablespoon capers, rinsed and drained
- 12 cherry tomatoes, halved
- 24 black or green California Ripe Olives, drained and halved
- Several sprigs of fresh thyme
1 Preheat the oven to 450°F. Set one oven rack in the upper third of the oven and another rack in the lower third.
2 Toast the bread: Spread the baguette slices in a single layer on a baking sheet. Place on the top rack of the oven and toast in the oven for 4 minutes. Remove from oven and let cool to room temp.
3 Line the baking dish with onions: Place the thinly sliced onions in a 9x13-inch baking dish and toss with 2 tablespoons of olive oil.
Spread the onions out over the bottom of the pan. Sprinkle with 1 tablespoon balsamic vinegar and 1/4 teaspoon of salt.
Pour 1 cup of water or stock onto the onions in the baking dish.
4 Prepare the mini peppers: Cut the peppers in half lengthwise through the stem. Pull out and discard the seeds.
Set pepper halves on top of the sliced onions in the baking pan, hollow side facing up.
5 Stuff the peppers: Sprinkle 3 to 4 capers inside each pepper half. Distribute the olive halves and cherry tomato halves among the pepper cavities.
Place a few thyme sprigs among the peppers.
Drizzle the peppers with the remaining 1 tablespoon of olive oil.
6 Roast the peppers: Cover the pan with aluminum foil. Bake in the oven at 450°F for about 30 minutes on the lower shelf of the oven, or until the peppers are soft.
Remove the foil and return the peppers to the top shelf of the oven for 10 to 15 more minutes, until the peppers have started to brown at the edges.
If the bottom of the pan becomes dry at any time, pour in a little more water. Remove the pan from the oven and remove the thyme sprigs.
7 Assemble the crostini: Move one of the peppers aside in the roasting dish to reveal the caramelized onions below. Place a forkful of onions on a toasted bread slice. Top with the pepper. Repeat until all of the crostini have been topped with peppers.
Serve warm or at room temperature. Garnish the platter with a few sprigs of fresh thyme.