Slow cookers have a heating element generally on the bottom of the unit which heats the ceramic insert, which in turn heats whatever is inside it. They are designed to heat liquids, and the foods cook because the heat is convected around by the liquid.
Slow cookers are not designed to cook without liquid, and I personally would not try cooking dry things in them for the following reasons:
- Uneven heating: the heat in a slow cooker typically is from the bottom, if you put a casserole in you'll get a very cooked bottom and a very underdone top. Not good in a pie, where the top needs heat to form a crust
- Damage of the heating element/safety concerns: liquid has a high capacity for heat and cushions the heating of the element in the slow cooker, without liquid in it the element may overheat and cause damage, or maybe even (in some older units) start a fire
- Damage to the ceramic insert: the ceramic insert in slow cookers is designed to carry liquid, if you try to cook dry foods it is liable to crack
If you want to cook a casserole or a pie use an oven as it will deliver even heat all around. Use a slow cooker as it is intended.
That doesn't mean you cannot use a slow cooker to make bag-friendly meals. Simply make dishes where you can discard the juice without losing the majority of the flavor, reduce the liquid on the stove afterwards until it is more of a gravy so it won't leak out, or add a thickener to the sauce to stabilize it at lower temperatures.
Some ideas for you:
- beef brisket with vegetables - doesn't need the liquid, the flavor is in the food. Bring some nice crusty bread with it
- stew: you can remove the solids and then gently cook down some of the liquid until it is a gravy, or add a thickening agent like corn starch (corn flour to some). Add enough so it will gel up when cold, but turn back to a liquid (albeit thick) when heated
- lamb or chicken tagine: remove the liquid and use it to make some couscous. The couscous will absorb the flavors of the sauce and make a fabulous side. Reduce the rest to make it thicker and use it to add flavor to the solid ingredients and keep them moist