Bring a large pot of salted water to a boil. Add the whole wheat spaghetti and cook according to package instructions until al dente, about 8-10 minutes. Drain and set aside.
While the pasta cooks, heat olive oil in a large skillet over medium heat.
Add the minced garlic to the skillet and sauté for 1 minute until fragrant, stirring frequently to avoid burning.
Add the diced tomatoes, salt, black pepper, and red pepper flakes to the skillet. Cook for 7-8 minutes, stirring occasionally, until the tomatoes break down and form a chunky sauce.
Stir in the chopped basil leaves and lemon juice into the tomato sauce. Cook for an additional 1 minute.
Add the chopped baby spinach to the skillet and cook until just wilted, about 1-2 minutes.
Add the cooked spaghetti to the skillet and toss well to combine with the sauce and spinach.
Remove from heat and serve immediately.