This is an intensely flavorful ice cream. The color is a pretty pale tan, and the texture is either nubbly or silken depending on whether or not you choose to sieve it. The final flavor will be only as good as the nuts. Taste them before buying; they should be sweet.
The ice cream mixture can be refrigerated for a day or two before freezing. Once frozen, it holds nicely for two days.
Preheat the oven to 350°F. Move a rack to the middle position.
Spread the nuts on a baking sheet and toast until they turn pale brown beneath the skin, 10 to 12 minutes. Stir the nuts midway to ensure even toasting. Wrap the hot nuts in a clean, dry kitchen towel and rub vigorously to remove the skins. Don’t get hyper; a bit of skin is flavorful in the final mix.
Immediately after toasting, process the nuts in a food processor, running the machine for a minute or more to obtain a relatively smooth, oily paste. Add the half-and-half and sugar, then process a minute more to blend.
Should you wish a smooth-textured ice cream, strain the mixture through a fine-mesh sieve into a bowl. (I never bother; I like the tongue-feel of the nuts.)
Add the liqueur, then add the lemon juice in drops until the flavor peaks on your tongue.
Freeze in an ice cream maker according to the manufacturer’s instructions. Store with a piece of plastic wrap pressed directly on the surface. Let soften slightly before serving.