Peel, core and slice the apples into even pieces (around 8 slices per half), or dice if you prefer.
Put the slices of apple in a bowl as you cut them and add the lemon juice, tossing to spread the lemon on to all the pieces.
Add the vanilla paste and sugar to the apples and toss to mix.
Warm the butter in a medium-small skillet until just starting to brown. Add the apple slices and cook a few minutes on each side, turning relatively often to ensure they cook evenly. They are ready when they are tender and just starting to brown. Serve warm or room temperature.