Is it an actual brew kettle or just a pot? If a kettle, then drain your mash into a bucket (hopefully you have one available/free to use) and then mash. Depending on how much grain you're using, you could let it rest in a stainless bowl while you heat the sparge water. Or use a couple of pots to heat the sparge water and pour it right into the kettle once the mash has been drained.
This is why many of us have more than one kettle/pot to use. I had two 20qt and one 32 quart pots for use while doing BIAB. Migrating to the cooler MT means you still need more than one pot, but you just need the second one large enough to heat the sparge water. I drain the mash into the boil kettle and then use one of the 20qt pots to heat up the sparge water. When I go to 10 gallon batches, I'll just use my 60qt kettle for the mash water, and the 32qt for the sparge. Just a matter of making sure you balance the volumes properly (with the grain bill) to make sure you can do it.