Yotam Ottolenghi's potato and celeriac gratin with caper brown butter

Name: Yotam Ottolenghi's potato and celeriac gratin with caper brown butter
Cuisine: European, British
Ingredients: 80g unsalted butter; 1 1/2 tbs extra virgin olive oil, plus more for greasing; 2 medium (360g) onions, peeled, halved and sliced into thin half-moons; 5 garlic cloves, crushed; 1 1/2 tbs thyme leaves, plus 3 sprigs; 1 1/4 tbs white miso; 180ml dry white wine; large celeriac (800g), peeled and cut into 1cm-thick batons (600g peeled); 680g yellow-fleshed potatoes, peeled and cut into 1cm-thick batons; 3 tsp plain flour; 300ml chicken stock (or vegetable stock); 150g Gruyere, roughly grated; 2 tbs baby capers, drained and patted dry; 2 lemons; 1/4 lightly packed cup finely chopped fresh chives