In a saucepan, heat the oil and cook the garlic until it just starts to brown. Add the stock, add the saffron, and simmer for 5 minutes.
In a small bowl mix flour and butter until smooth then add to the stock and mix over high heat and bring to a boil to thicken.
Taste the sauce and season with salt and pepper to taste, add the parsley, remove from the heat and set aside.
Slice the potatoes into thin, evenly sized slices, add them to the casserole dish in layers (seasoning with salt and pepper in betweeb layers). Pour the stock evenly over the potatoes. Bake for 30 to 40 minutes or until the potatoes are tender and the top is golden brown. Serve hot.
When adding a flour mixture to thicken, it must come to a boil in order to thicken the sauce. Stir well and bring to a boil.
Always make sure your potatoes are sliced into similar thicknesses to enable an even cook.