Heat oven to 220C/200C fan/gas 7 and grease 2 x 15cm sandwich tins with solid bases. In a non-stick pan, gently cook the shallots in the oil until golden and softened. If they start to colour before softening, add a dash of water and cover the pan. Tip out the shallots and wipe out the pan.
Put the butter in the pan with the nuts, garlic, rosemary, maple syrup and vinegar. Season and allow to bubble until syrupy. Tip in the shallots and cook briefly, stirring to coat them. Spoon half the shallots and syrup into each tin, then add the beetroots, alternating them with the shallots as you go.
Cut the pastry in half, then roll out each piece on a lightly floured surface until just larger than the tins. Place the pastry over the vegetables and tuck it down the insides of the tin. Trim off any excess with scissors and pierce the middle of the pastry with a knife to make a hole for steam to escape. Bake for 15-20 mins until golden. Cool for a few mins in the tin, then turn out onto plates and top with the rocket.