To make the surf sauce; melt butter and stir in garlic - cook for 2 mins, don't allow garlic to brown. Then add flour, stirring all the time & cook for a minute or so then whisk in wine with a hand whisk.
Heat slowly, whisking the whole time to keep sauce smooth, slowly adding milk until all used (use less or more according to thickness). Heat and keep whisking, simmer until it is to the thickness you require. Then stir in the cream, herbs and seasonings. Bring back to simmer and bring to consistency you prefer. Then add prawns and simmer until prawns are cooked.
To make the mashed potato; boil the potato and sweet potato till tender, then mash together with the seasonings. Stir in the milk and butter until smooth.
For the asparagus; and boil or steam the asparagus until tender/crisp - about 5 minutes or so, depending on their size - then toss through seasonings & olive oil. Keep the mashed potatoes and the asparagus warm.
When you're ready to eat; cook the steaks to your taste medium rare is best of course! I like to BBQ them as they can all be cooked at once and I prefer the chargrill taste.
To assemble; place a dollop of mash in the centre of each plate and lay 4 spears of asparagus to the side of each. Place the steak on the mash and ladle over some prawn sauce - as much or as little as you like but usually half a ladleful of sauce with 6/7 prawns each person is good.
You can make the garlic white sauce ahead of time and then reheat when your ready to cook the prawns.