1. In Dutch Baby, heat the olive oil over a medium-high flame. Add the fennel, leek, and white scallions. Cook, stirring occasionally, until soft and starting to lightly brown, about 7 minutes. Add the garlic and cook for another few minutes until becoming golden.
2. Add the potatoes, stock, and salt. Bring to a boil, then reduce the heat to medium-low and simmer, uncovered, until the potatoes are soft enough to pierce with a fork, about 15 minutes.
3. Meanwhile, trim the asparagus by removing the tough bottom inch, or any section that is thick, white, and hard to cut through, and discard. Roughly chop all but 4 asparagus spears. Finely chop the remaining spears and set aside for garnish.
4. Once the potatoes are tender, stir in the roughly chopped asparagus and all but 2 tablespoons of the green scallions. Cook for 2 minutes until the asparagus is al dente and vibrant green.