Green beans in mushroom sauce

I make these as a side dish in winter. Goes great with roasts, baked chicken, ham...

2 cups frozen green beans
1/2 cups frozen mushrooms
1 Tbsp butter
1 Tbsp flour
1/2 cup milk
1/4 tsp beef bouillon
freshly ground pepper
1/4 tsp rosemary

Melt butter in a non-stick frying pan over medium heat. Stir in flour, then half the milk. Stir until smooth, then add remaining milk. Once smooth, add bouillon and rosemary. Add in frozen mushrooms and green beans. Cook, stirring occasionally, until the beans are bright green, about 10 minutes. Season with pepper.