Bring a large pot of water to a rolling boil, season generously with kosher salt, and cook spaghetti until al dente. Reserve 0.75 cup pasta water, then drain.
In a large skillet over medium heat, warm olive oil and melt butter. Add onion and cook until translucent, about 3 minutes. Stir in garlic and cook until fragrant, about 0.5 minute.
Stir in gochujang and cook, stirring, for 1 minute to bloom and caramelize the paste slightly.
Add soy sauce, honey, and rice vinegar. Stir to combine into a smooth base.
Pour in heavy cream and 0.5 cup of the reserved pasta water. Whisk until the sauce is silky and begins to simmer, 2 to 3 minutes.
Add the drained spaghetti and toss to coat. If needed, splash in more reserved pasta water until the sauce clings glossy to the noodles.
Off the heat, stir in toasted sesame oil and half of the Parmesan. Season with fine sea salt, black pepper, and red pepper flakes to taste.
Plate and garnish with scallions, sesame seeds, and the remaining Parmesan. Serve immediately.
Notes
For a lighter version, swap half of the heavy cream with whole milk. Adjust heat by increasing or decreasing gochujang and red pepper flakes. Leftovers keep up to 2 days; reheat gently with a splash of water.