Steam broad beans for 3 minutes. Cut potatoes into cubes, cook in boiling water for 10 minutes. 
Melt butter in saucepan, stir in flour and cook for 1 minute, stirring constantly. Remove from heat; gradually stir in milk, stock powder, garlic, cheddar cheese and thyme. Return pan to heat and bring to boil again, stirring constantly; cook for 1 minute. 
Combine beans and potatoes in 1 litre ovenproof dish; pour sauce over vegetables. Sprinkle on the parmesan cheese and parsley. Bake at 180°C for 30 minutes or until cheese is browned lightly.