This is a very forgiving, flexible recipe, so feel free to adjust as you go.
3-4 sweet potatoes (I used Japanese white-fleshed)
chicken stock to cover (home-made, yum)
1 tart apple
2 cloves garlic
(1 shallot, optional)
1/2 Tbsp favorite dry herb mix (I used Herbs de Provence)
1 tsp dried sage (or 2 tsp fresh sage)
2 strips bacon
1) Cut bacon into thin strips. Fry over medium heat until all fat has rendered out and bacon pieces are crispy. Reserve 1 Tbsp bacon pieces and approximately 1 Tbsp bacon fat in medium saucepan; place rest of bacon on paper towel to drain, and save remaining bacon fat for another purpose.
2) Mince garlic (and shallot, if using) finely. Fry over medium-low heat with bacon, stirring constantly, until just fragrant. Do not brown.
3) Add peeled + cubed sweet potatoes, and peeled, cored, cubed apple. Cover with chicken stock; if you don't have enough stock to cover, add enough water until just covered. Add herbs.
4) Bring to boil; immediately reduce heat to lowest. Leave to simmer, uncovered, 1-2 hours to allow flavors to meld. Turn off heat. Puree carefully with immersion blender; adjust seasonings to taste. Serve topped with remaining bacon.
This recipe is endlessly adjustable: you can use any starchy or non-starchy vegetable (broccoli, kale, yams, carrots, tomatoes, celery root, etc). If you want to do a "curried"-type soup, omit the dry herbs and add 1/2 can coconut milk after cooking as well as 1 Tbsp of your favorite curry or masala; you can also add a hot pepper of your choice to the sauteeing stage, and a bit of grated fresh ginger. If you want a "cream"-type soup, leave in the herbs, add 1/2 can coconut milk, and maybe double the garlic (or add roasted garlic, yum), and proceed as usual.
This recipe depends very strongly on using a good, well-balanced home-made stock, simmered low and slow with lots of veggies for flavor. I also recommend using organic veggies for most pronounced flavor.