Start with preparing your portobello mushrooms by giving them a wipe with a damp paper/kitchen towel and removing the stems.
In a shallow dish that will fit all four mushroom caps mix together ingredients for the marinade.
Place the mushrooms in the dish (top of the caps down), brush the other side with generous amount of marinade and let them sit and soak up the flavours for 15 minutes.
After approximately 15 minutes flip your portobello mushroom caps top side up and brush with some more marinade. Let them sit in the dish for another 15 minutes if you have time or give them 5 minutes and get grilling.
Drizzle some rapeseed oil onto the grill pan and make sure it is distributed evenly (I like to brush it on).
Put your pan onto a medium heat and place your portobello mushroom caps bottom side down in the pan.
Grill for 5-7 minutes pressing down gently with a spatula from time to time to get a little bit of caramelisation going as well as to get that flat patty shape.
Flip your mushrooms over and repeat the process brushing some extra marinade on your mushroom caps as you go.