Cook the fettuccine (100 g) per package directions; drain.
Season prawns (300 g) with salt (1 tsp) and pepper (1/2 tsp), then toss with the flour (2 tbsp) (2 tbsp added in two batches) to coat evenly.
Melt butter (1 tbsp) and olive oil (1 tbsp) in a large skillet over medium-high. Cook prawns in a single layer, flipping once, until golden, 2-3 min per side. Remove.
Deglaze with chicken broth (120 ml), whisking, ~5 min. Lower to medium, whisk in cream (120 ml), cook until bubbly and thickened ~5 min. Adjust with cajun seasoning (1/2 tsp), salt and pepper.
Stir drained pasta (100 g) and prawns (300 g) into the sauce.