Heat oven to 190C/fan 170C/gas 5. Oil a 23 x 33cm Swiss roll tin. Cut a rectangle of baking parchment 4cm larger all round than the tin. Fold in a 2cm strip all round, making a diagonal cut into each corner.
For the roulade, melt the butter in a large pan. Add the garlic and fry for 1 min. Add the flour and cook
for 1 min, stirring constantly. Gradually beat in the milk. Bring to the boil, stirring constantly, until the sauce is thick and smooth. Remove from the heat and stir in half the cheese. Season, then allow to cool slightly. Beat in the egg yolks and parsley. Whisk the egg whites to stiff, but not dry peaks, then gently fold into the cheese sauce. Pour into the tin, tipping the tin so that the mixture fills all the corners. Bake for 15 mins until risen and golden.
Meanwhile, heat the oil and butter in a large wok or frying pan and stir-fry the spring greens and garlic for 4-5 mins until tender. Season.
Sprinkle the remaining cheese over a sheet of baking parchment just larger than the tin. Turn the roulade out onto the parchment, then peel off the lining paper. Trim the crispy edges off the roulade.
Use a slotted spoon to remove the spring greens from the garlicky butter, then spread over the roulade leaving a 2.5cm space at one short end uncovered. Roll up the roulade from the short end with no filling on it, using the paper to help. Serve the roulade immediately, drizzled with any remaining garlicky butter.