Heat a large skillet over medium-high heat and melt the butter until foaming.
Add the flour and cook, stirring constantly, until incorporated and smooth, about 1 minute.
Add the minced garlic and cook, stirring, until fragrant, about 30 seconds.
Pour in the heavy cream, then stir in the lemon juice, Italian seasoning, and sun-dried tomatoes; bring to a gentle simmer for about 2 minutes, reducing heat if the sauce boils vigorously.
Add the shrimp to the sauce and cook, stirring occasionally, until the shrimp are opaque and cooked through, about 4–6 minutes depending on size.
Stir in the baby spinach and sliced basil and cook until the spinach wilts, about 1–2 minutes; season with salt and pepper to taste.
Serve immediately, optionally squeezing additional lemon juice or sprinkling grated Parmesan if desired.