Cook cauliflower in boiling salted water for 2-3 minutes. It needs to be crunchy. Put in ice water. Drain and dry as much as possible.
Do the same for the broccoli.
Heat half-and-half in a saucepan over low heat (do not boil). Add blue cheese and cook, stirring constantly with a wire whisk until cheese melts. Don't melt all the cheese - leave some chunks. Remove from heat. Wisk in mayo.
Pour blue cheese mixture over the veggies, tossing gently.
Put in good bowl and sprinkle top with green onions and pecans.