I fry onions in a pan on the cooktop until they are translucent and then add chopped garlic and cook for a couple more minutes. Note: frying the onions changes the flavour. A stew with unfried onions doesn't taste right to me. Frying onions can take about 10 minutes so I usually cut up the meat and veges while they are frying. Also, I usually fry a double amount of onions and garlic and freeze half for my next dish to save time.
Put the onions and garlic into the slow cooker, then turn the heat up in the pan a little to quickly sear the beef, turning it to make sure all sides are brown. This should only take a couple of minutes. If you want spices in your stew you can dust the beef with the spices before throwing it into the pan.
The beef I use is a cheap cut called chuck steak here in Oz but it might be called something else where you are.
Note: Don't use expensive frying steak like sirloin because that works best when fried quickly e.g. on the barbie, so use the type which is best for slow cooking.
Put the seared beef into the slow cooker. If you are feeling so inclined you could fry up the chopped veges for a few minutes but that's not essential. I just find that frying them brings out the flavours. I use carrot, orange sweet potato, potato as my main veges but I usually put the potatoes in about half way through the cooking.
Put the veges in the slow cooker, add enough stock to just cover the lot and press the on button and walk away. I use liquid stock from a packet and I use half stock and half hot water.
There are variations on flavours.
If you are using herbs, put them in towards the end of cooking or the flavour will be lost but spices can go in at the beginning.
If you are using Italian herbs and spices, add a bit of tomato paste to bring out the flavour of the meat.
Or, beef bourguignon/beef in red wine is excellent. You can fry some bacon first, then after you fry the beef, take it out and throw in a good glug of red wine to lift the flavours off the pan.
Beef and Guinness is similar. Both make great fillings for pies. I often make pies with leftovers the next day using store-bought frozen puff pastry for the top.
One of my favourite beef dishes uses Moroccan spices and near the end of cooking - maybe an hour before the end in a slow cooker, or about 20 minutes on the stove or in the oven - throw in about a cupful of prunes which have been soaked in boiling water. Put the prune water in as well. I buy a spice mix here called Arabic seven spices: paprika, ground cumin, ground coriander, ground cloves, ground nutmeg, ground cinnamon, ground cardamom or there are different variations available.
Moroccan lamb and apricot stew is also one of my faves. Same idea as the beef with prunes. Soak some dried apricots in boiling water and add the apricots and water near the end of the cooking time.