Finely chop (or roughly grate) the squash, onion and garlic. Heat some oil in a skillet over medium-low heat. 

When the oil is hot, add the vegetables and the thyme (stems and all) and let everything brown for 8-12 minutes. Scrape the bottom of the skillet when things begin to stick so it doesn't burn. 

Lower the heat a bit. If the skillet seems dry, add water, one deciliter at a time. Let the mixture cook for another 10-15 minutes, stirring and scraping occasionally.

At this point, check the softness of the vegetables by squeezing them with the back of a spoon. If they are still firm and do not begin to smash when you squeeze, let them cook for another 5-10 minutes.

When the vegetables are soft, remove the stems of the thyme (most of the leaves should have detached from the stem by this point). Mix in the salt, lemon juice and lemon zest, then use the back of a spoon to smush everything together. If you're not getting to a spread-like texture, add water a little bit at a time. Taste and adjust the seasoning if necessary.

Take the spread off the heat. You can serve it immediately, or store it in a container in the fridge.