I wrote it before, and I will write it once more: The (proper) dairy is my No1 point of disagreement with mainline Paleo, with lentils being the No2. Lentils have too much iron, manganese, and folate, nutrients that are sorely missed when going too-low carb. They are essentially the best kinds of legumes in terms of nutrition. Unfortunately, they also have a lot of anti-nutrients: loads of lectins, to be exact.
In the olden days, beans would only be eaten while they’ve been previously fermented (soy too). But in the fast-pacing modern days we live in, convenience rules, so people stopped fermenting foods. According to an experiment carried out by researchers, a 24 to 36 hours fermentation of lentils gets rid of most of the lectins! At the end of the fermentation, the lectins and anti-nutrients surviving are not more than the ones found on a carrot or spinach. So in my opinion, Paleo fanatics who are adamant about the no-legumes rule, need to ease up. Just like with dairy, there are exceptions to the rule.

Lentils and Peas by Photobunny Earl. Licensed under the Creative Commons BY-NC-ND 2.0.
So, how to ferment lentils? There are two ways to do this, either via pickling, or via lacto-fermentation (preferred).
Common Steps (for 1 cup of lentils):
A. Lentils are usually cross-contaminated with grains (since they grow in grass fields), so you must go through your raw lentils and remove anything that doesn’t look like a lentil.
B. Wash the lentils thoroughly using your palms, and sift-strain them.
Acidity Fermentation:
1. Place in a bowl, and add double the amount of water than that of lentils. The water must be slightly warm, around 30 C. For this type of fermentation, any kind of water will do, but filtered is best.
2. Add 1 tablespoon of raw vinegar, or the juice of a small lemon into the water. Stir, and cover (but not air-tight).
3. After 12 hours, strain the water away, and repeat steps 1 & 2 (every 12 hours). Ferment for 24 to 36 hours.
Lacto-fermentation:
1. Place in a bowl, and add double the amount of water than that of lentils. The water must be slightly warm, but no more than 25-30 C. For this type of fermentation, non-tap water must be used. Use either filtered, or bottled water. The good bacteria we will use to ferment, can’t survive on tap water.
2. Add 1.5 tablespoons of plain yogurt, or preferably, 1/4 cup of home-made goat kefir. Stir, and cover (but not air-tight).
3. After 12 hours, strain the water away, and repeat steps 1 & 2 (every 12 hours). Ferment for 24 to 36 hours.
Drain and wash them again, then cook your lentils according to your recipe (although probably they will require less cooking time).