This is not a dish by itself but rather a tasty addition to pizza, pasta sauce, etc. Or try it on bruschetta. Carmelizing mellows the flavor of fennel and brings out its sweetness, just as is true of onions.
Making this does not take a lot of effort, but it does take a lot of time, so plan to make a large batch sometime when you will be available to check in on it periodically over a few hours. The finished product will keep in the fridge for at least a few days, or you can freeze it in small quantities for later use. I've suggested quantities here for the fennel and onions, but you can increase it as you see fit (and as fits in your pot). The quantities below will fill a Dutch oven to the top at the beginning of the cooking process.
2 Tbsp olive oil
2 very large or 4 medium fennel bulbs, coarsely chopped with cores removed
6 medium onions, sliced lengthwise
Salt to taste
Heat the olive oil in a Dutch oven or other large pot. Add the fennel and onions and stir to coat with olive oil. Saute over medium heat for a few minutes, then reduce the heat to low and partially cover. Cook over low heat, stirring occasionally, for 2-3 hours, until the fennel and onions have been reduced to a soft mass and have turned a nice golden brown. Add salt to taste. You can continue to cook them to a darker shade if you like, but if you choose to do so, be sure to watch them and stir more often to prevent them from going too far.
Makes 2-3 cups.