One of the best trick with leeks is the following, if you really want the flavour to shine:
melt a little butter, add the chopped (or sliced) leeks and seasoning and sweat gently over a low heat for about 30mins. Dont let it get too hot or they will burn; just the merest hint of a sizzle is all that is needed; and no lid needed at all (the flavour intensifies with no lid). Butter has a real affinity with leeks.
This REALLY does draws out the flavour bigtime - even on an otherwise bland variety of leek. But patience with this cooking is the key. But then if they have grown for so many months this is no time at all!
This can now be eaten as it is or as a base for one of the following favourites of mine for a more impressive dish:
(1) Mix in some dijon mustard and a dollop of creme fraiche (great side dish with a roast either as is, or put in a shallow dish in the oven/grill to brown on top)
or
(2) Mix the cooked leeks with 3-4 beaten eggs, a dollop of cream or creme fraiche, and some cheese (gruyere is fab for this, as is mature cheddar) and put in a pastry case - bake as for a quiche. Heavenly warm or cold.
And another thing I do with leeks earlier in the year is sow an extra packet in a raised bed or the equivalent with the sole purpose of using the shoots (when they are a lot thinner than they would be for planted out properly to grow as full leeks) in a salad - bursting with leek/oniony flavour but not too overpowering. This year I managed to do 3 sowings a month or two apart to have them in salads through to autumn.