I keep some items in my pantry and elsewhere to be prepared always.
There are the sweet staples, such as flour, sugar, confectioner’s sugar, vanilla extract, butter, and eggs, of which you never want to run out. To these I would add frozen puff pastry (see my post “Frozen Puff Pastry”), almond flour (see my posts “Gluten-Free Goodies” and “King of the Castle”), dark chocolate (see my posts “Mousse au Chocolat” and “Gateau au Chocolat”), and heavy cream (see my posts “Gosh, Chocolate Ganache” and “A Thousand Kisses Deep”).
On the savory side, in addition to salt and pepper and the French trinity of onions, carrots, and celery, include bacon (see my post “Bacon”), extra-virgin olive oil and vinegar (see my post “A Simple Vinaigrette”), dried lentils (see my post “Lentilles du Puy”), spaghetti (see my posts “Carbonara” and “Clam Up”), Arborio rice and dried porcinis (see my posts “Mushroom Risotto” and “Coq au Vin”), and a frozen chicken (see my posts “Roasted Chicken” and “Chicken Dinners”). Parsley dresses up any dish, so buy a bunch, or better yet grow your own (see my posts “Herb Bouquets” and “Patio Pots”).