Chop broccoli and cauliflower. On medium high heat, put cauliflower in pot with 50 ml water. Cover with lid and cook for about 10 minutes. Next add broccoli, on top and reduce heat to medium, cooking with lid on - for another 10 minutes, or until vegetables are tender and broccoli is bright green. Drain excess water in sauce and leave vegetable mixture on low heat.
To make sauce:
Begin cooking sauce while cooking vegetables. Melt butter in small saucepan on medium heat. Add flour and mix continuously for about one minute. Once the roux (flour mixture) is pasty, add milk, broth, and salt/pepper (to taste) very slowly. Cook until thickened and add lemon juice last.