Prepare the vegetables and put it all in a pot with a little water, and bring to a boil. Parboil only, for about 2 - 3 minutes, until bright green but still a little crunchy. Add salt and drain in a colander or sieve.
Heat the butter and olive oil in a skillet, then add the chopped onion and bacon. Use fairly high heat, and stir often with a wooden spoon, until onions and bacon are soft, not brown or crisp.
Turn heat off, but keep on the hob while adding lemon juice, sugar (or substitute) and mustard. Mix all well with the back of a spoon.
Put the vegetables in a heated oven dish, then ladle over the bacon-lemon topping.
There won't be much sauce, as my idea was to flavour but not obscure the crunchy taste of the fresh veggies.
If the vegetables have cooled while you made the sauce topping, cover the dish with foil and leave in a warming oven to warm through -- but not for too long!