Heat olive oil in a large pan over medium-high heat, then add beef mince and cook for 5 minutes until golden edges form and meat sizzles.
Add diced onion and chopped capsicum, sauté for 2–3 minutes until tender with a slight bite.
Stir in minced garlic and Cajun seasoning, cooking for 1 minute until fragrant.
Pour in diced tomatoes, beef stock, and penne; bring to a rolling boil.
Reduce heat, cover, and simmer for 12 minutes, stirring occasionally to prevent sticking.
Remove lid and stir through grated cheddar until the sauce turns glossy and smooth.
Season with salt and pepper to taste, then top with chopped parsley and serve immediately.
Notes
Adjust the Cajun seasoning: reduce to 1½ Tbsp for milder heat and increase to 2½ Tbsp if you prefer a bolder kick.If the sauce is too thin, simmer uncovered for a few more minutes to let excess liquid evaporate and achieve a thicker consistency.