Whole-roasted celeriac with tahini herb sauce

Ingredients

• 1 large celeriac , scrubbed and rinsed (about 1 kg)
• 3 tbsp unsalted butter , softened and divided
• 1 1/4 tsp salt , divided
• 1/2 cup chopped toasted walnuts
• 1/4 cup panko bread crumbs
• 1/2 cup parsley leaves , divided
• 1 cup basil leaves
• 2 garlic cloves , crushed
• 1/4 cup water
• 1/4 cup tahini
• 1/4 cup lemon juice
• 2 tbsp canola oil
• 1 tbsp honey
• 1 cup microgreens , for garnish