Bring a large pot of lightly salted water to a boil. Cook angel hair pasta in the boiling water, stirring occasionally, until tender yet firm to the bite, 4 to 5 minutes. Drain.
Melt butter in a frying pan over medium-high heat. Saute mushrooms, onions and garlic until onions are translucent, about 5 minutes. Add wine, stock, tomatoes and parsley. Bring sauce to a boil. Stir and reduce heat to low. Simmer, covered, for 10 minutes.
Place prawns and flour in a resealable plastic bag; shake and mix until prawn is well coated. Stir prawns into the sauce. Cook until prawns are opaque, about 5 minutes. Pour sauce and prawns over the pasta; squeeze lemon juice on top. Season with salt and pepper.