Pulse thyme, flour, butter, and lard in a processor until the mixture looks like breadcrumbs.
Gradually add cold water to form a soft dough, then chill for 30 minutes.
Cook the red onion jam ingredients over low heat until softened and combined.
Preheat the oven to 200°C and heat a baking tray inside.
Divide and roll pastry to line four 10cm tart tins.
Blind bake the tarts with baking beans for 10 minutes, then without for five minutes.
Mix crumbled goats' cheese with egg yolk, cream, salt, and pepper.
Fill pastry cases with onion jam and cheese mixture.
Bake for 8 minutes until bubbling and browned.
Whisk oil and lemon juice, then toss with rocket leaves.
Top tarts with the rocket salad and serve.