- Scrub or peel the sweet potato. Trim the ends and cut the potato in half lengthwise. Cut each half lengthwise into ¼-inch-thick slices, the slices crosswise into ¼-inch cubes.
- Finely chop, press, or grate enough garlic to measure 1 teaspoon [2 tsp].
In a large frying pan over medium-high heat, warm 2 tablespoons oil until hot but not smoking. Add the sweet potato, season with salt and pepper, and cook, stirring occasionally, until starting to soften, 6 to 8 minutes. Add the remaining onion, season with salt and pepper, and cook, stirring occasionally, until starting to soften, 2 to 3 minutes. Stir in the garlic and tomato paste and cook until fragrant, about 30 seconds.
Add ¼ cup [½ cup] white wine (from your pantry), if using, and cook until thickened slightly, 1 to 2 minutes. Add the tomatoes and 1½ cups [2½ cups] fish poaching liquid. Bring to a boil, reduce to a simmer, cover, and cook until the flavors come together and the vegetables are just tender, 4 to 6 minutes.
While the stew simmers, prepare the parsley.