Roasted beets: If you are roasting your own beets, trim the stem, drizzle with olive oil, wrap in foil, and bake at 350 for ~45 minutes. Then allow to cool and peel.
Dressing: With a mortar and pestle, pound the garlic with a pinch of salt until it has the consistency of a smooth paste and transfer to a medium bowl. Mix the two oils in a bowl. Slowly add the oil to the bowl while whisking constantly. If it gets too thick, add a couple drops of lemon juice. Keep whisking until it is creamy, opaque, and thick. Taste and adjust seasoning and lemon juice. Add both the mustards, and adjust the consistency with a little lemon juice and water so it drizzled easily. Adjust seasoning.
Mix the leeks, fennel, and 2 tablespoon of olive oil and season with salt. Add the beets, and pour over the chopped lettuce. Drizzle with dressing and serve immediately!