Split the spaghetti squash in half lengthwise. Scoop out seeds. Place on a baking dish with cut side down and bake for 40 to 50 minutes.
Remove spaghetti squash from oven. When cool enough to handle, use a fork to gently scrape around the edge of the squash to shred the pulp into strands.
Toss strands of spaghetti squash with lemon juice, salt, and freshly grated pepper.
Combine arugula, garlic, sliced almonds, parmesan cheese, and salt in a food processor. Give it a few pulses. With machine running, gradually add olive oil through the feed tube of food processor. Process until smooth.
To serve, toss flavored spaghetti squash with desired amount of pesto. Garnish with extra arugula and sliced almonds.