Slice the bacon strips in half, making 4 short strips. Saute these over medium-low heat until desired level of crispy.
Drain most of the bacon fat, leaving a teaspoon or so in the pan. Saute the red onion until it starts going soft, about 2 minutes. Add the spinach and pepper and cook, stirring occasionally, until spinach is wilted.
Lightly butter one side of each piece of bread. Assemble sandwich: bread, half of the brie, bacon, vegetables, brie, and bread.
Cook on a panini press until bread is golden brown and brie is melting, 2-3 minutes. If you don’t have a panini press, simply use the saute pan, flipping the sandwich once (optional: flatten the sandwich by laying a smaller pan or flat-bottomed plate on top).