For the herb purée, bring salted water to boil in medium saucepan. Add watercress, spinach, parsley, basil, tarragon and chervil leaves, gently stirring for 5–10 seconds.
With slotted spoon, transfer herbs to reserved ice water bath. Drain blanched herbs, pat dry with paper towels, and roughly chop.
Transfer chopped herbs to blender and purée until smooth; set aside.
For the toasted bread crumbs, heat butter and olive oil in medium sauté pan over medium heat.
Add panko bread crumbs, Espelette chili powder, salt and white pepper, and cook until golden brown.
Remove from heat and stir in lemon zest and chopped dill; set aside.
For the chowder, melt butter in large stock pot over medium heat. Add chopped bacon and cook browned and crisp, 8 minutes.
Add diced onion, carrot, celery and fennel, season with salt and pepper, and sauté until vegetables are tender and translucent, 10 minutes. Add minced garlic and cook until fragrant, 1 minute.
Reduce heat to low, add flour and cook, stirring continuously, 5 minutes.
Add Pernod and white wine and cook until liquid has reduced by half, 5–7 minutes.
Add vegetable stock and increase heat to simmer. Add potatoes and cook until tender, 10–12 minutes.
Stir in heavy whipping cream and simmer until heated through. Just before serving, add oysters and simmer, 3–5 minutes.
Ladle chowder into bowls, garnish with a swirl of herb purée and top with toasted bread crumbs.
Herb purée and chowder can be made 1 day ahead and stored in separate airtight containers in the refrigerator. Toasted bread crumbs can be made 3 days ahead and stored in airtight container at room temperature.