--------------- Serves 4 Warm butter in a large skillet (I like to use cast iron) on the stovetop over medium-high heat.
Using a sharp knife, carefully cut into the thickest part of the chicken breast (but not completely through the chicken) to create a butterfly effect when opened up that will allow chicken to cook faster and more evenly. Combine Cajun seasoning in a small bowl. Sprinkle both sides of chicken liberally with the seasoning. Add chicken to hot pan and cook, uncovered, until chicken is cooked through, about 4 minutes per side, flipping chicken at least once. The chicken will take on a gorgeous dark golden colour from the seasoning.
Transfer chicken to a large plate and cover loosely with foil.
Warm the same pan, adding a little olive oil. Toss in the onion and shiitake and stir-fry, scraping up any leftover brown bits from the chicken, until vegetables are just beginning to soften. Add tomatoes, warming until they start to break down and accumulate surrounding juices. Cut chicken breasts in half and serve with vegetables overtop.