If you are trying to cut down on the amount of oil you use, put a teaspoon of sunflower oil in a non-stick pan, and wipe it around with a piece of kitchen towel - which will leave a thin coating of oil on the pan while soaking up any excess.
Stir the veg around to get as much oil on them as possible, then as soon as they look as if they might stick, add a tablespoon of water, or stock. If needed, repeat as the veg cook.
Sunflower oil has a high smoke point, a fairly neutral flavour, and as oils go, is relatively healthy.
If you don't have a steamer, put a flat bottomed bowl in the pan upside down and fill with water up to the bowl base, and stand your veg on the base of the bowl. A steamer is better, but an inverted bowl will work.
Water turns to steam at 100c, whereas oils cook at around 200c - the smoke-point of sunflower oil is approximately 230c.
I don't really go with this notion of sweating to remove moisture, because oil will very quickly seal the surface of veg as the sugars come to the surface and caramelise. TV chefs often say: "I've got some caramelised onions here, which I have been sweating down on a low heat!" "Sweating is a loosely used term for frying at low temperature, but because of the temperature of a pan's surface even on low heat, using oil will quickly seal the surface. If it didn't, the oil would simply soak into the veg.
Steaming cooks veg in a relatively low temperature moist environment, which means the veg doesn't soak up water as with boiling, which also tends to wash out colour and nutrients.
Green leafy veg is usually better when steamed, if the recipe allows.