Bring a 5- to 6-quart pot of salted water to boiling. In a large skillet heat 2 tablespoons of the oil over medium-low heat. Add bread crumbs to hot oil; cook about 3 minutes or until crumbs are crisp and golden brown, stirring frequently (reduce heat and stir constantly if bread crumbs are becoming too dark). Stir in 1 clove of the minced garlic; cook and stir until garlic is fragrant. Transfer mixture to a small bowl. Stir in 1/4 cup of the Parmesan cheese and the lemon peel; set aside.
In the same large skillet heat the remaining 2 tablespoons oil over medium-high heat. Add onion and salt; cook about 3 minutes or until onion is tender, stirring occasionally. Stir in the 1 teaspoon thyme and the remaining 2 cloves minced garlic; cook and stir for 30 seconds. Add cauliflower, broth, and capers; cover and cook about 10 minutes or until cauliflower is tender. Stir in lemon juice and pepper.
Meanwhile, in the large pot cook spaghetti according to package directions, except cook for 2 minutes less than the time given on the package. Drain spaghetti and return to large pot.
Add cauliflower mixture to spaghetti. Cook about 5 minutes more or until spaghetti is al dente. Divide spaghetti mixture among six shallow serving bowls. Sprinkle with the bread crumb mixture and the remaining 1/4 cup Parmesan cheese. If desired, garnish with additional snipped fresh thyme or thyme sprigs.