From Cook's Illustrated: The Best Make-Ahead Recipe.
Why this recipe works:
After some preliminary testing for our slow-cooker lentil and Swiss chard stew recipe, we decided that large brown and green lentils did the best job of retaining their texture, shape, and flavor through the all-day simmer. As the lentil and Swiss chard stew recipe progressed, we realized it needed more flavorful components to boost the lentils and chard, which, admittedly, tasted pretty thin. After testing a variety of additional ingredients, we most liked the earthiness of mushrooms—in particular, meaty portobello mushrooms pumped up with dried porcini.
PREP-AHEAD TIPS: You can store the following ingredients together, with the exception of the chard leaves:
1. Cook the onion-chard stem mixture as described in step 1, then transfer the mixture to an airtight container and refrigerate; prep the chard leaves and refrigerate.
2. Prep the carrots and refrigerate.
3. Prep the portobellos and refrigerate.
4. Pick over and rinse the lentils and refrigerate.
Be sure to choose large green or brown lentils and avoid red lentils or lentils du Puy, both of which will overcook. Carefully sort through the lentils to remove small stones and pebbles and then rinse.