Preheat the oven to 450 degrees F. Slice the bell pepper in half and remove the stem and seeds. Place the halves cut side down on a lined baking sheet. Bake for 25 to 30 minutes until the skin is blistered and blackened.
Remove the peppers from the oven and peel off the skin with your fingers. If the pepper is too hot to handle, run it under cold water as you remove the skin. Slice the pepper into thin strips and toss with 1 teaspoon olive oil and the minced garlic. You can roast the pepper up to a few days in advance and keep it in the fridge until you want to make the sandwich. I like to roast a few peppers so I can make several sandwiches.
While the peppers are roasting, prep the other ingredients. Slice the onion and cucumbers. Toss the arugula with the balsamic reduction and the other 1 teaspoon of olive oil. Toast the bread to your liking.
When it's time to make the sandwich, divide the Boursin cheese between the two slices of bread and spread. Top one slice with the cucumber slices, roasted peppers, red onions, and arugula. Place the other slice of bread on top, cut in half, and enjoy!
To save prep time, make the roasted red peppers in advance and slice the onions and cucumbers. Store them in an airtight container in the fridge until it's time to assemble the sandwich. I like to make 3 roasted red peppers and slice an entire red onion and cucumber so that I have enough to make several sandwiches.
The sandwich is best eaten right away, but you can definitely pack it in a container to bring for lunch at work or school. It tastes good warm or cold.