Preheat the oven to 180°C/Fan 160°C/Gas 4. Line an ovenproof baking dish with baking paper.
Pour the cream into a small saucepan. Add the garlic and thyme, place the pan over a medium heat and bring the cream to a boil. Reduce the heat to stop the cream scorching, but continue to cook until the liquid has reduced by half (about 7–8 minutes).
Remove the thyme sprigs. Layer the potatoes, kohlrabi, thinly sliced bacon and dots of the butter with the reduced cream and season with salt and pepper in between the layers. Bake for 35–45 minutes, until the vegetables are tender when tested with a wooden skewer or point of a knife.
Meanwhile make the garlic crumbs. Heat the olive oil in a large frying pan over a medium heat. Add the breadcrumbs and cook, stirring continuously, until they start to turn golden. Add the butter and continue cooking until the crumbs are golden brown. Tip the crumbs on to a tray, stir through the garlic granules and onion powder and season with salt and pepper. To serve, scatter the hot gratin with the garlicky crumbs, then the chopped herbs and grated Parmesan.