Preheat oven to 350°. In a large bowl, toss berries with tapioca. In a small bowl, combine honey, lemon juice, and 1 tbsp. boiling water. Stir to dissolve honey. Add to berries and toss to combine. Put in an 8- by 8-in. baking pan and set aside.
Spread hazelnuts on a baking sheet and toast in oven until medium golden brown, 10 to 15 minutes. Rub nuts in a kitchen towel to remove skins; let nuts cool. In a food processor, pulse nuts until finely ground. Add flour, sugar, and salt and pulse to combine. Add butter and pulse until mixture forms a thick dough. Stir in oats. Drop in flattened 1-tsp. chunks over berries. Bake until topping is brown and berries are bubbling, about 1 hour. Let cool to set, 30 minutes.