For the creamy lemon-basil vinaigrette:
1) Combine lemon juice, zest, mayonnaise, honey, salt, pepper, basil and olive oil in a blender and blend until smooth. This can be made 1 hour in advance and refrigerated.
For the mushrooms:
1) Preheat the griddle to high.
2) Place the mushrooms in a large bowl, add the rapeseed oil and salt and pepper and toss to coat.
3) Skewer mushrooms, 4 per skewer. Place the mushrooms on the griddle, until golden brown and slightly charred, 4 to 5 minutes. Turn over and continue to griddle until just cooked through, 3 to 4 minutes longer. Remove from the griddle to a cutting board and quarter.
4) Place the celery and parsley in a large bowl, add half of the vinaigrette and toss to combine, season with salt and pepper. Top with the mushrooms and drizzle with more of the vinaigrette. Squeeze lemon juice over the salad and serve.