Cook the steak: lightly salt and pepper, then grill over medium-high heat until medium rare. Alternately, sear in a hot skillet with a splash of olive oil for approximately 2 minutes per side. Let rest 10 minutes and then thinly slice.
Heat a large skillet over medium heat and add the butter, white onion, bell pepper, and a pinch of salt and pepper.
Sauté for about 20 minutes until caramelized, stirring frequently.
Heat the oven to broil and line a baking sheet with parchment paper.
Gently rub oil on the slices of French baguette and place on the prepared baking sheet.
Broil for about 2 minutes until crispy, flip, and broil for another 1-2 minutes. Keep an eye on things, because these can toast more quickly, depending on your oven!
To assemble, spread a thin layer of Raspberry Marionberry Fruit Spread onto the crostini. Follow with desired amount of blue cheese, a thin slice of cooked steak, and top with the caramelized onions and peppers.