Warm the olive oil with the garlic slices in it in a relatively small skillet/frying pan (but big enough to hold the steak) over a medium heat. Warm until the garlic becomes aromatic and is sizzling but not browning. Remove from the heat and allow to cool slightly.
Mix the wine and mustard in a dish just a little bigger than your steak. Add the oil and garlic once it has cooled a bit and mix. Don't wash out the skillet as you will use it to cook the steak.
Place the steak in the wine-oil mixture and turn so it is coated. Leave to marinate at room temperature for around 20 minutes turning once or twice. The meat will start to change color a little.
Warm the skillet/frying pan to a medium-high heat. Remove the steak from the marinade - don't throw it out - and grind a little salt and pepper on either side of the steak.
Add the steak to the warm pan and cook for around 3-5 minutes on each side, depending on thickness and level of doneness you are looking for. Once you've cooked it for almost as much as it needs on the second side, add some of the marinade mixture (around 2tbsp if you have ½lb steak, around double for 1lb) and ½tbsp parsley and allow it to cook and reduce for a minute or two.
Remove the steak from the heat, transfer the steak to a board and let it rest for 5min. Slice and serve with the heated marinade mixture over the top then top with the remaining parsley and the crumbled blue cheese.
Notes
You can use the same amount of marinade for up to double the amount of steak, ie 1lb/450g. This will give you more servings. If increasing steak amount, then also increase the amount of blue cheese.