Slice lettuce crosswise into 1-inch strips and transfer into a large bowl.

Add vegetable oil to a large skillet, and place over medium-high heat.

When the oil is hot, add shallots, green onions, and garlic, and cook and stir until shallots start to turn translucent, about 2 minutes.

Add brown sugar, rice vinegar, fish sauce, tamarind paste, soy sauce, chili sauce, and Sriracha, and stir to combine. As soon as the sauce starts to boil, turn off heat and set aside until needed.

Meanwhile, bring a large pot of salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally, until barely tender yet firm to the bite, about 11 minutes, or about 30 seconds less than package directions.

Use tongs to transfer noodles into the pan of sauce, and cook, stirring, over medium-high heat for about 30 seconds.

Transfer noodles and sauce to the bowl of lettuce, and toss until everything is evenly combined, about 1 minute, or longer if you prefer the lettuce more wilted.