Whisk together the lemon juice, oils and mustard for the dressing first so that it is ready to use.
Peel the celeriac, removing all the outer layer and any dark, hairy indents. Peel and core the apple. Coarsely shred both and toss with the dressing to prevent browning.
Cook the bacon as you prefer (eg under broiler, in oven/in pan - this can be done ahead) and cut into slices. Roughly chop the parsley.
Add the bacon, pumpkin seeds and parsley to the apple and celeriac and toss to combine well. Serve.
Notes
You can use a few different kinds of apple in this but I suggest using a good flavored, sweet-tart eating apple with a good crispness such as honeycrisp, pink lady, Mcintosh or envy.