Petey,

Leeks are a type of onion but I don't think the flavor is as strong as an onion is. I have a recipe for Cauliflower Leek Puree and it is quite good.
I have used onions in place of the leeks when I don't have leeks though.

Cauliflower Leek Puree
Leeks are relatively low in carbs (for a member of the onion family) and have a fresh flavor which pairs well with cauliflower. The single leek used here turns this dish into a close double for mashed potatoes. 5.5 carbs

1 head cauliflower, separated into florets
1 leek, white and 1" green, well washed, cut into 1/2" slices
1 teaspoon salt, divided
3 tablespoons butter
3 tablespoons cream
1/8 teaspoon nutmeg (optional)
salt and pepper

1. Cook cauliflower and leeks in lightly salted boiling water 15 minutes, until very tender. Drain; return veggies to pot and toss over high heat to thoroughly remove excess moisture.

2. Place half the vegetables in food processor with half the butter and cream. Process until smooth. Repeat with remaining vegetables, butter, and cream. Mix in nutmeg; season to taste with salt and pepper.