Cut the passion fruit in half and scoop out the pulp and seeds into a small bowl. Put the shallot, garlic, honey, and lemon juice, and Worcestershire in a blender and whiz until smooth. Add the vinegar.
Pour over the passion fruit. Stir and press with a rubber spatula to rinse the seeds and separate them from the pulp. Pour through a medium sieve, pressing to release as much pulp as you can. Stir in the arrowroot, heat until thickened, and pour over spinach or other salad greens.