With a large, sharp knife, carefully split squash in half lengthwise and remove seeds and membranes with a spoon. Brush the flesh of each squash half evenly with ½ teaspoon olive oil, then sprinkle each half with ¼ teaspoon salt and 1/16 teaspoon of black pepper.
Place squash, skin side up, in a 9x13 baking dish. Microwave on full power for 15-20 minutes, until the tops give slightly under pressure when gently pressed (checking for doneness every couple of minutes at the end of cooking time.) Let sit about 10 minutes until cool enough to handle (or handle with oven mitts), and then use a fork to shred squash into a large serving bowl. (If your squash is a bit overcooked and a little watery, you can place it in a colander for a few minutes to drain before using a slotted spoon to transfer it to the bowl. Read further tips on this in the full recipe post.)
Meanwhile, while squash is cooking, toast walnuts in a small skillet over medium-low heat, stirring frequently to prevent burning. Remove from heat as soon as the walnuts smell fragrant or begin to lightly brown. Set walnuts aside and carefully wipe out skillet.
In the same skillet, over medium-low heat, melt butter and cook, gently swirling the pan or stirring occasionally, until the butter has foamed and begun to turn a light golden-brown. (This will take about 3-4 minutes, varying greatly depending on how powerful your burners are.) Add the sage and continue browning the butter a little further, about 1 minute, just until the butter has turned a richer golden-brown with little specks of milk solids in the bottom of the pan. Remove the butter from the heat and stir in remaining ¾ teaspoon salt and ⅛ teaspoon black pepper.
Add toasted walnuts and browned butter sauce to spaghetti squash and stir gently to thoroughly combine. Serve immediately from the bowl, or (if you'd like to present this as a stuffed spaghetti squash), by piling the squash mixture back into one of the spaghetti squash "shells."
by Two Healthy Kitchens
Browning butter, toasting walnuts, and dealing with watery spaghetti squash: Be sure to read our tips and recommendations in the post and look through the step-by-step photos for suggestions on all these steps of the recipe.