Heat the milk and cream (if using) in a saucepan over medium heat until just about to boil. Stir occasionally to prevent scorching.
Remove from heat and stir in lemon juice or vinegar. Watch as the curds separate from the whey within a few minutes.
Line a fine-mesh strainer with cheesecloth and place over a bowl. Pour the mixture through and let strain for several hours or overnight in the refrigerator.
Transfer the strained curds to a food processor or blender. Add salt and blend until smooth and creamy, scraping down sides as needed.
Store in an airtight container in the refrigerator for up to 1 week. Use as a spread on bagels or customize with your favorite mix-ins.